After checking into some heavy problems with some commercial SMTP mailers and
SMTP proxy servers it was time to check on the standards and I found that
there's quite a problem there.
The world is filling up with mail servers that are only available thru batched
smtp (waiting to be released by the ETRN command in SMTP (rfc1985). Looking at
the below part of RFC0974 (last paragraph of page 5, inserted below to make
sure we're reading the same) a mailer MUST try the preference with the lowest
value (in 9916433467503f the batched SMTP cases the actual host behind the
dial-up link) and MAY try to go on to higher valued preferences if the first
Since an average batched host link is down most of the time, the message is
almost doomed to be undeliverable, if a mailer only implements the 'MUST'
features. Although it's very easy to say that a 'off-line' mailhost should not
have an MX record, it's still the reality today and with many ISPs their
standard way of creating batched smtp.
So, in conclusion, I'd propose a change for the part that a server MAY try the
second host in line to MUST try at least the second host in line to ensure that
all batched SMTP mail is delivered.
Looking forward to your comment,
********* start quote **********
If the list of MX RRs is not empty, the mailer should try to deliver the
message to the MXs in order (lowest preference value tried first). The mailer
to attempt delivery to the lowest valued MX. Implementors are encouraged
to write mailers so that they try the MXs in order until one of the MXs accepts
message, or all the MXs have been tried. A somewhat less demanding system,
in which a fixed number of MXs is tried, is also reasonable. Note that multiple
MXs may have the same preference value. In this case, all MXs at with a
given value must be tried before any of a higher value are tried. In addition,
special case in which there are several MXs with the lowest preference
value, all of them should be tried before a message is deemed undeliverable.
********** end quote ***********
Research & Development Dept.
Azlan Network Services
SkyNet MailServer v1.00Beta
Copyright 1998 A.J. Louw. All Rights Reserved.