mdw@git.distorted.org.uk Git - qmail/blob - addresses.5

   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)