Commit | Line | Data |
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2117e02e MW |
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) |