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