OWA OA Publishing failures

[AndreasY]

For some resaon my ISA breaks my publishing rule all of a sudden (for OWA
and OutlookAnywhere). Then I make other adjustments and it works and a few
weeks later it breaks again with certain errors.

For example, my OA publishing rule was very simple and some time ago it
stopped working. I reconfigured it by following some Technet articles so I
used NTLM authentication, Delegation from AD for ISA to the Mailserver and
it worked. A few weeks later it stopped again and now I see error: that ISA
failed to delegate and make sure that SPN http/owa.mydom.com is defined in
AD. However in AD the SPN is the name of the mailserver
(mailsrv.localdomain.com).

Is this correct for SPN?
Could anyone explain why could the rule work for 3 weeks and then suddenly
it decides that something is wrong?