1 Quick Mail Transfer Protocol (QMTP)
2 D. J. Bernstein, djb@pobox.com
8 The Quick Mail Transfer Protocol (QMTP) is a replacement for the
9 Simple Mail Transfer Protocol (SMTP). QMTP eliminates any need for
10 end-of-line scanning between hosts with the same end-of-line
11 convention. It features automatic pipelining and chunking, 8-bit
12 transmission, prior declaration of the message size, and efficient
13 batching. It is designed to be very easy to implement.
15 QMTP is supported by the qmail-qmtpd and maildir2qmtp programs in the
18 In this document, a string of 8-bit bytes may be written in two
19 different forms: as a series of hexadecimal numbers between angle
20 brackets, or as a sequence of ASCII characters between double quotes.
21 For example, <68 65 6c 6c 6f 20 77 6f 72 6c 64 21> is a string of
22 length 12; it is the same as the string "hello world!". Note that
23 these notations are part of this document, not part of the protocol.
28 A QMTP client connects to a QMTP server, as discussed in section 7,
29 over a reliable stream protocol allowing transmission of 8-bit bytes.
31 Protocol outline: the client sends one or more packages; after each
32 package, the server sends back some responses.
34 The client begins by sending a package. A package contains a mail
35 message, an envelope sender address, and one or more envelope
36 recipient addresses. See section 4 for the format of a package.
38 When the server sees the end of the package, it sends back a series
39 of responses, one response for each envelope recipient address, in
40 the same order as given by the client. The server is not permitted to
41 change the order under any circumstances, even if two addresses are
42 the same. See section 5 for the format of a response.
44 The server is not permitted to send any portion of its responses to a
45 package until the client has sent the final byte of the package. The
46 client is permitted to close the connection before sending the final
47 byte of the package; in this case, the server must throw away the
48 package without attempting to deliver the message. However, the
49 server must not throw away previously accepted messages.
51 The client does NOT need to wait for a server response before sending
52 another package. The server must NOT throw away incoming data when it
53 sends a response. It is the client's responsibility to avoid
54 deadlock: if it sends a package before receiving all expected server
55 responses, it must continuously watch for those responses. The server
56 is permitted to delay its responses if further data has already shown
57 up from the client; while it is delaying responses, it must not pause
58 to wait for further data for the client.
60 The server is permitted to close the connection at any time, although
61 high-quality servers will try to avoid doing so. Any response not
62 received by the client indicates a temporary failure.
64 A QMTP session should take at most 1 hour. Both sides are expected
65 to close the connection after this time.
70 In this document, an ``8-bit mail message'' means a sequence of
71 lines. Each line is a string of zero or more 8-bit bytes.
73 A message is called ``safe'' if none of its bytes are <0a>.
75 Implementation note: Here is the intended interpretation of text
76 files as messages under some current operating systems. Under DOS, a
77 message is stored on disk as
79 first line, <0d 0a>, second line, <0d 0a> ... <0d 0a>, last line.
81 Under UNIX, a message is stored on disk as
83 first line, <0a>, second line, <0a> ... <0a>, last line.
85 Notice that both of these encodings are reversible for safe messages.
87 In practice, it is very common for the last line to be empty. Many
88 existing utilities refer to the last line as a ``partial line'' and
89 ignore it whether or not it is empty.
94 A package is the concatenation of three strings:
96 first, an encoded 8-bit mail message;
97 second, an encoded envelope sender address;
98 third, an encoded series of encoded envelope recipient addresses.
100 Each envelope address is a string of 8-bit bytes. The interpretation
101 of addresses depends on the environment in which QMTP is used and is
102 outside the scope of this document. Each address is encoded as a
103 netstring, as discussed in section 6. The series of encoded recipient
104 addresses is in turn encoded as a netstring.
106 A message is encoded as a string of 8-bit bytes in one of two ways:
108 Encoding #1 is <0d>, the first line, <0d 0a>, the second line,
109 <0d 0a>, the third line, ..., <0d 0a>, the last line.
111 Encoding #2 is <0a>, the first line, <0a>, the second line, <0a>,
112 the third line, ..., <0a>, the last line.
114 This string of 8-bit bytes is in turn encoded as a netstring, as
115 discussed in section 6.
117 Every server must be prepared to handle encoding #1 and encoding #2.
118 A server must not reject a message merely because of its encoding.
120 Implementation note: The intent of encoding #1 and encoding #2 is to
121 allow very straightforward handling of text files under DOS and UNIX
122 respectively. The programmer can print <0d> or <0a> and then simply
128 Each response is a nonempty string of 8-bit bytes, encoded as a
129 netstring. The first byte of the string is one of the following:
131 "K" The message has been accepted for delivery to this envelope
132 recipient. This is morally equivalent to the 250 response to
133 DATA in SMTP; it is subject to the reliability requirements
134 of RFC 1123, section 5.3.3.
136 "Z" Temporary failure. The client should try again later.
138 "D" Permanent failure.
140 The remaining bytes are a description of what happened. It is
141 expected that the description, when interpreted as UTF-2 characters,
142 (1) will be human-readable, (2) will not repeat the envelope
143 recipient address, and (3) will not include formatting characters
144 other than <20>. However, these expectations are not requirements,
145 and the client should be ready for arbitrary bytes from the server.
147 Descriptions beginning with <20> are reserved for future extensions.
148 In descriptions not beginning with <20>, the character "#" must not
149 appear except in HCMSSC codes.
151 A server must NOT accept a safe message unless it can store the
152 message without corruption. More precisely: if the encoded message
153 sent by the client matches the encoding of some safe message M, then
154 acceptance means that the server is accepting responsibility to
155 deliver M to the envelope recipient. (There is at most one
156 possibility for M, since encodings are reversible on safe messages.)
157 Deletion of nulls is NOT permissible; a server that deletes nulls
158 must reject any message containing nulls. Folding of long lines and
159 high-bit stripping are also NOT permissible.
161 Servers are permitted to change unsafe messages.
166 Any string of 8-bit bytes may be encoded as [len]":"[string]",".
167 Here [string] is the string and [len] is a nonempty sequence of ASCII
168 digits giving the length of [string] in decimal. The ASCII digits are
169 <30> for 0, <31> for 1, and so on up through <39> for 9. Extra zeros
170 at the front of [len] are prohibited: [len] begins with <30> exactly
171 when [string] is empty.
173 For example, the string "hello world!" is encoded as <31 32 3a 68
174 65 6c 6c 6f 20 77 6f 72 6c 64 21 2c>, i.e., "12:hello world!,". The
175 empty string is encoded as "0:,".
177 [len]":"[string]"," is called a netstring. [string] is called the
178 interpretation of the netstring.
183 QMTP may be used on top of TCP. A QMTP-over-TCP server listens for
184 TCP connections on port 209.
189 A client opens a connection and sends the concatenation of the
193 "Received: (qmail-queue invoked by uid 0);"
194 " 29 Jul 1996 09:36:40 -0000" <0a>
195 "Date: 29 Jul 1996 11:35:35 -0000" <0a>
196 "Message-ID: <19960729113535.375.qmail@heaven.af.mil>" <0a>
197 "From: God@heaven.af.mil" <0a>
198 "To: djb@silverton.berkeley.edu (D. J. Bernstein)" <0a>
200 "This is a test." <0a> ","
201 "24:" "God-DSN-37@heaven.af.mil" ","
202 "30:" "26:djb@silverton.berkeley.edu," ","
205 "From: MAILER-DAEMON@heaven.af.mil" <0d 0a>
207 " Hate." <22> "The Quoting" <22>
208 "@SILVERTON.berkeley.edu," <0d 0a>
209 " " <22> "\\Backslashes!" <22>
210 "@silverton.BERKELEY.edu" <0d 0a>
212 "The recipient addresses here could"
213 " have been encoded in SMTP as" <0d 0a>
215 " RCPT TO:<Hate.The\ Quoting@silverton.berkeley.EDU>" <0d 0a>
216 " RCPT TO:<\\Backslashes!@silverton.berkeley.edu>" <0d 0a>
218 "This ends with a partial last line, right here" ","
220 "83:" "39:Hate.The Quoting@silverton.berkeley.edu,"
221 "36:\Backslashes!@silverton.berkeley.EDU," ","
223 The server sends the following response, indicating acceptance:
225 "21:Kok 838640135 qp 1390,"
226 "21:Kok 838640135 qp 1391,"
227 "21:Kok 838640135 qp 1391,"
229 The client closes the connection.