Just looking for some clarification on how SMTP and DNS work together to deliver a mail to a working mail server.

Say mydomain.com has the following MX records:

MX 10 mail1.mydomain.com
MX 20 mail2.mydomain.com
MX 30 mail3.mydomain.com

If my mail server (nothing to do with above domain) is sending to simon@mydomain.com it will try mx pref 10 first. Now does it simply establish a port 25 connection and if it fails move onto the next mx pref, or is it a little more complicated than that?

Also, if the same mail server wanted to send a mail to david@mydomain.com, would it make the same mistake and try mx pref 10 even though it was down 10 secs ago? Im guessing it would but would like to know for sure.

If anyone has any decent links on the subject then feel free to post em.

Cheers,
Simon.

Yes I know the MX records arent fully qualified before some geek points it out :D

Simon.

The RFC is what you want, http://ftp.rfc-editor.org/in-notes/rfc2821.txt describes the use of DNS by SMTP.

Ok for once reading an RFC wasnt that bad :D

This is the section I was after:

When the lookup succeeds, the mapping can result in a list of
alternative delivery addresses rather than a single address, because of multiple MX records, multihoming, or both. To provide reliable mail transmission, the SMTP client MUST be able to try (and retry) each of the relevant addresses in this list in order, until a delivery attempt succeeds. However, there MAY also be a configurable limit on the number of alternate addresses that can be tried. In any case, the SMTP client SHOULD try at least two addresses.

Cheers Steve,

Simon.