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1 .TH addresses 5 |
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2 .SH "NAME" |
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3 addresses \- formats for Internet mail addresses |
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4 .SH "INTRODUCTION" |
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5 A |
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6 .B mail address |
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7 is a string of characters containing @. |
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8 |
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9 Every mail address has a |
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10 .B local part |
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11 and a |
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12 .B domain part\fR. |
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13 The domain part is everything after the final @. |
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14 The local part is everything before. |
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15 |
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16 For example, the mail addresses |
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17 |
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18 .EX |
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19 God@heaven.af.mil |
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20 @heaven.af.mil |
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21 @at@@heaven.af.mil |
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22 .EE |
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23 |
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24 all have domain part |
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25 .BR heaven.af.mil . |
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26 The local parts are |
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27 .BR God , |
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28 empty, |
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29 and |
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30 .BR @at@ . |
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31 |
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32 Some domains have owners. |
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33 It is up to the owner of |
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34 .B heaven.af.mil |
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35 to say how mail messages will be delivered to addresses with domain part |
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36 .BR heaven.af.mil . |
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37 |
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38 The domain part of an address is interpreted without regard to case, so |
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39 |
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40 .EX |
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41 God@heaven.af.mil |
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42 .br |
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43 God@HEAVEN.AF.MIL |
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44 .br |
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45 God@Heaven.AF.Mil |
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46 .EE |
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47 |
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48 all refer to the same domain. |
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49 |
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50 There is one exceptional address that does not contain an @: |
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51 namely, the empty string. |
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52 The empty string cannot be used as a recipient address. |
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53 It can be used as a sender address so that |
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54 the real sender doesn't receive bounces. |
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55 .SH "QMAIL EXTENSIONS" |
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56 The |
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57 .B qmail |
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58 system allows several further types of addresses in mail envelopes. |
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59 |
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60 First, an envelope recipient address without an @ is interpreted as being at |
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61 .IR envnoathost . |
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62 For example, if |
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63 .I envnoathost |
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64 is |
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65 .BR heaven.af.mil , |
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66 the address |
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67 .B God |
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68 will be rewritten as |
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69 .BR God@heaven.af.mil . |
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70 |
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71 Second, the address |
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72 .B #@[] |
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73 is used as an envelope sender address for double bounces. |
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74 |
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75 Third, envelope sender addresses of the form |
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76 .I pre\fB@\fIhost\fB-@[] |
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77 are used to support variable envelope return paths (VERPs). |
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78 .B qmail-send |
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79 will rewrite |
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80 .I pre\fB@\fIhost\fB-@[] |
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81 as |
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82 .I prerecip\fB=\fIdomain\fB@\fIhost |
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83 for deliveries to |
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84 .IR recip\fB@\fIdomain . |
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85 Bounces directly from |
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86 .B qmail-send |
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87 will come back to |
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88 .IR pre\fB@\fIhost . |
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89 .SH "CHOOSING MAIL ADDRESSES" |
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90 Here are some suggestions on choosing mail addresses for the Internet. |
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91 |
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92 Do not use non-ASCII characters. |
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93 Under RFC 822 and RFC 821, |
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94 these characters cannot be used in mail headers or in SMTP commands. |
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95 In practice, they are regularly corrupted. |
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96 |
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97 Do not use ASCII control characters. |
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98 NUL is regularly corrupted. |
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99 CR and LF cannot be used in some combinations |
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100 and are corrupted in all. |
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101 None of these characters are usable on business cards. |
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102 |
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103 Avoid spaces and the characters |
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104 |
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105 .EX |
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106 \\"<>()[],;: |
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107 .EE |
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108 |
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109 These all require quoting in mail headers and in SMTP. |
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110 Many existing mail programs do not handle quoting properly. |
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111 |
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112 Do not use @ in a local part. |
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113 @ requires quoting in mail headers and in SMTP. |
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114 Many programs incorrectly look for the first @, |
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115 rather than the last @, |
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116 to find the domain part of an address. |
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117 |
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118 In a local part, |
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119 do not use two consecutive dots, a dot at the beginning, or a dot at the end. |
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120 Any of these would require quoting in mail headers. |
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121 |
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122 Do not use an empty local part; it cannot appear in SMTP commands. |
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123 |
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124 Avoid local parts longer than 64 characters. |
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125 |
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126 Be wary of uppercase letters in local parts. |
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127 Some mail programs (and users!) will incorrectly convert |
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128 .B God@heaven.af.mil |
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129 to |
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130 .BR god@heaven.af.mil . |
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131 |
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132 Be wary of the following characters: |
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133 |
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134 .EX |
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135 $&!#~`'^*|{} |
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136 .EE |
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137 |
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138 Some users will not know |
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139 how to feed these characters safely to their mail programs. |
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140 |
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141 In domain names, stick to letters, digits, dash, and dot. |
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142 One popular DNS resolver has, |
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143 under the banner of security, |
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144 recently begun destroying domain names |
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145 that contain certain other characters, |
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146 including underscore. |
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147 Exception: A dotted-decimal IP address in brackets, |
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148 such as |
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149 .BR [127.0.0.1] , |
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150 identifies a domain owned by whoever owns the host at that IP address, |
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151 and can be used safely. |
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152 |
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153 In a domain name, |
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154 do not use two consecutive dots, |
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155 a dot at the beginning, |
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156 or a dot at the end. |
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157 This means that, |
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158 when a domain name is broken down into components separated by dots, |
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159 there are no empty components. |
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160 |
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161 Always use at least one dot in a domain name. |
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162 If you own the |
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163 .B mil |
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164 domain, |
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165 don't bother using the address |
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166 .BR root@mil ; |
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167 most users will be unable to send messages to that address. |
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168 Same for the root domain. |
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169 |
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170 Avoid domain names longer than 64 characters. |
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171 .SH "ENCODED ADDRESSES IN SMTP COMMANDS" |
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172 RFC 821 defines an encoding of mail addresses in SMTP. |
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173 For example, the addresses |
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174 |
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175 .EX |
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176 God@heaven.af.mil |
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177 .br |
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178 a"quote@heaven.af.mil |
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179 .br |
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180 The Almighty.One@heaven.af.mil |
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181 .EE |
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182 |
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183 could be encoded in RCPT commands as |
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184 |
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185 .EX |
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186 RCPT TO:<God@heaven.af.mil> |
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187 .br |
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188 RCPT TO:<a\\"quote@heaven.af.mil> |
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189 .br |
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190 RCPT TO:<The\\ Almighty.One@heaven.af.mil> |
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191 .EE |
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192 |
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193 There are several restrictions in RFC 821 |
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194 on the mail addresses that can be used over SMTP. |
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195 Non-ASCII characters are prohibited. |
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196 The local part must not be empty. |
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197 The domain part must be a sequence of elements separated by dots, |
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198 where each element is either a component, |
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199 a sequence of digits preceded by #, |
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200 or a dotted-decimal IP address surrounded by brackets. |
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201 The only allowable characters in components are |
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202 letters, digits, and dashes. |
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203 Every component must (believe it or not) |
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204 have at least three characters; |
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205 the first character must be a letter; |
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206 the last character must not be a hyphen. |
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207 .SH "ENCODED ADDRESSES IN MAIL HEADERS" |
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208 RFC 822 defines an encoding of mail addresses |
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209 in certain header fields in a mail message. |
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210 For example, the addresses |
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211 |
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212 .EX |
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213 God@heaven.af.mil |
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214 .br |
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215 a"quote@heaven.af.mil |
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216 .br |
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217 The Almighty.One@heaven.af.mil |
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218 .EE |
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219 |
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220 could be encoded in a |
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221 .B To |
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222 field as |
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223 |
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224 .EX |
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225 To: God@heaven.af.mil, |
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226 .br |
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227 <@brl.mil:"a\\"quote"@heaven.af.mil>, |
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228 .br |
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229 "The Almighty".One@heaven.af.mil |
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230 .EE |
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231 |
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232 or perhaps |
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233 |
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234 .EX |
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235 To: < "God"@heaven .af.mil>, |
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236 .br |
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237 "a\\"quote" (Who?) @ heaven . af. mil |
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238 .br |
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239 , God<"The Almighty.One"@heaven.af.mil> |
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240 .EE |
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241 |
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242 There are several restrictions on the mail addresses that can |
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243 be used in these header fields. |
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244 Non-ASCII characters are prohibited. |
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245 The domain part must be a sequence of elements separated by dots, |
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246 where each element either (1) begins with [ and ends with ] |
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247 or (2) is a nonempty string of printable ASCII characters |
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248 not including any of |
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249 |
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250 .EX |
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251 \\".<>()[],;: |
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252 .EE |
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253 |
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254 and not including space. |
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255 .SH "SEE ALSO" |
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256 envelopes(5), |
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257 qmail-header(5), |
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258 qmail-inject(8), |
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259 qmail-remote(8), |
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260 qmail-smtpd(8) |