On Fri, February 13, 2015 10:57, Ken Smith wrote:

Hi All,

I'm just wanting to check that my understanding of the settings is correct as my web searches are finding a lot of dated information.

If I want a Centos 6 sendmail system act as the secondary MX for domain bbbbb.co.uk do I just add a

Connect:bbbbb.co.uk RELAY

statement into /etc/mail/access and restart sendmail

Obviously I have the DNS MX records for the domain are already established.

I've been getting "/config error/: /mail loops back to me/ " errors.

I think I may be stumbling into a variant of cname problem where the hostname as far as the sendmail machine is concerned is aaaaa.com but the DNS setting for the secondary MX is smtp1.bbbbb.co.uk.

They both resolve to the same IP but when sendmail looks up the MX records for bbbbb.co.uk it will find smtp.bbbbb.co.uk and smtp1.bbbbb.co.uk listed and it may relay the mail off to smtp1.bbbbb.co.uk without recognising that aaaaa.com = smtp1.bbbbb.co.uk. Am I on the right track here, as I then just need to change the secondary MX setting in DNS to aaaaa.com?

When I set up secondary MX services in Sendmail (and Postfix) then I always use the direct address feature of the domain routing table and avoid looking up MX RRs altogether. After all, if the mail arrived here it is a good bet that the main MX is off-line (or this is SPAM/UCEM but that is another issue).

So assuming that the primary MX host is mx10.example.com and the secondary is mx40.something.else then with Sendmail the file /etc/mail/mailertable on mx40.something.else should contain something like this:

example.com. esmtp:[mx10.example.com] .example.com. esmtp:[mx10.example.com]

The [] brackets prevent MX lookups and just routes the message traffic directly to mx10.example.com as soon as a connection can be made.

This prevents the most common source of mail loops where the primary is off-line and so any mail is bounced back to the backup MX, which just happens to be the host that just sent it, thus causing the loop.