The Mail Exchanger Protocol Switch (MXPS)
D. J. Bernstein, email@example.com
Mail messages today are transferred through the Simple Mail Transfer
Protocol (SMTP). One can imagine other protocols that achieve the
same results as SMTP but that, for example, use the network more
The Mail Exchanger Protocol Switch (MXPS) lets other protocols
compete with SMTP. A receiver can announce its support for another
protocol while operating properly with MXPS-ignorant senders. A
sender can check for support, with no overhead, while operating
properly with MXPS-ignorant receivers.
All receivers must support SMTP, i.e., must be able to receive
messages via SMTP. Similarly, all senders must be able to send
messages via SMTP.
2. The protocol switch
MXPS abuses the preference field of MX records. A protocol is
assigned to each possible preference.
SMTP is assigned to preferences 0 through 10000.
The initial MXPS experiment will involve preferences between 12800
and 13055 inclusive. These preferences are sliced into 16 portions:
12800, 12816, 12832, 12848, 12864, ..., 13040: slice #0 (SMTP)
12801, 12817, 12833, 12849, 12865, ..., 13041: slice #1 (QMTP)
12802, 12818, 12834, 12850, 12866, ..., 13042: slice #2
12815, 12831, 12847, 12863, 12879, ..., 13055: slice #15
Preferences in slice #0 are assigned SMTP. Preferences in slice #1
are assigned the Quick Mail Transfer Protocol (QMTP). Preferences in
the remaining slices may be assigned protocols in the future.
A receiver must support the protocol assigned to its preference. More
precisely, if an MX record points to domain D, and the MX preference
is assigned protocol P, then every host listed as an A record for D
must support protocol P.
When a sender, following the procedure outlined in RFC 974 (and
modified by RFC 1123), attempts to deliver a mail message as
specified by that MX record, it may use protocol P instead of SMTP.
If it does not support protocol P, it may treat the attempt as a
temporary failure and go on to the next MX record. However, the
sender must not skip every MX record.
MX records must never use unassigned preferences. A sender may treat
an unassigned preference as referring to SMTP.
A.EXAMPLE.ORG IN MX 12801 A.EXAMPLE.ORG
B.EXAMPLE.ORG IN MX 12801 A.EXAMPLE.ORG
IN MX 12816 C.EXAMPLE.ORG
A sender with a message for A.EXAMPLE.ORG will try A.EXAMPLE.ORG by
QMTP. If it does not support QMTP, it will try SMTP instead. Note
that A.EXAMPLE.ORG must support both QMTP and SMTP.
A sender with a message for B.EXAMPLE.ORG will try A.EXAMPLE.ORG by
QMTP, then C.EXAMPLE.ORG by SMTP. If it does not support QMTP, it may
try SMTP instead of QMTP, or it may skip A.EXAMPLE.ORG.
Some of the above requirements might be violated if current
MXPS-ignorant domains use any preferences above 10000. Mail could be
unnecessarily rejected if any existing MXPS-ignorant domains have a
best-preference MX above 10000. I do not know any examples of such
3. Protocol requirements
MXPS operates purely at the link level. It does not change the
fundamental nature of Internet mail.
The function of a mail transfer protocol is to transmit a message, as
described below, together with an envelope sender address and one or
more envelope recipient addresses.
A recipient address is a sequence of characters---i.e., nonnegative
integers---including an ASCII @ (64). It is parsed as box@dom, where
dom does not contain an @. The interpretation of box is up to the
hosts listed as MX records for dom. A sender address may contain an
@, in which case it is also of the form box@dom; or it may be a
special address, such as the empty string.
A mail message is structured as a sequence of lines. A line is a
sequence of characters. Every mail transfer protocol must be able to
transmit all sufficiently short boring mail messages. A boring mail
message is one where (1) no line has more than 80 characters and (2)
each character is either 9 or between 32 and 127 inclusive. Note that
RFC 1341 defines a mechanism for encoding a message with characters
between 0 and 255 inclusive as a boring mail message of similar
The receiver must indicate, for each recipient address, either
acceptance, permanent rejection, or temporary rejection of the
message. Acceptance means that the receiver has taken responsibility,
in the sense of RFC 1123, section 5.3.3, for delivering the message
to that recipient. Rejection means that the receiver will not deliver
the message to that recipient.
Mail transfer protocols may vary in many details, such as line
encodings, the means of expressing acceptance or rejection, the
maximum number of allowable recipients per envelope, the encoding of
envelope addresses, the nature of optional protocol extensions, etc.
4. Security considerations
MXPS does not change the following facts: An attacker who can subvert
the Domain Name System can steal or forge mail. An attacker who can
subvert TCP/IP can also steal or forge mail.