addresses.5 (6101B)
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)