The usual suspect for confusion with most anything on OS X or OS X Server that's using IP and that needs security or integrity is an errant DNS set-up.
DNS errors have caused this problem with Postfix before, too.
I'd avoid a mail server using multicast DNS; that .local TLD used on the host name. (Per the RFCs, SMTP mail servers are configured with DNS A (machine) records, and various other SMTP servers interoperating with your mail server can and variously will depend on that A record as part of spam filtering and related tasks. And a .local isn't an A record that another server can resolve.)
Launch Terminal.app and issue the usual (diagnostic) tool to verify your local DNS set-up.
sudo changeip -checkhostname
It'll prompt for an administrative password, then display some configuration information and an indication that DNS is valid and no changes are required, or that there are host or DNS errors.
I had this problem on a Mini SL Server for ages, tracked it to whenever the DSL line went down, this fix courtesy of this forum fixed the problem for me, as, to date, even if the DSL line has gone down the hostname remains correct !
"sudo scutil --set HostName applecrackers.co.uk [replace with your hostname]" [without the quotes!]
You MUST be Logged in as "root" !
HTH ** Mike **