Mail server has just stopped working. No errors in any log. Help!

Question

I am running OS X Server 3.0.2 on Mavericks 10.9.1. Today at around 14:30 my Mail server, which has been working fine for over a year and for nearly a month under Server 3.0.2 just stopped working! I have tried stopping and starting Mail in Server.app and via ServerAdmin but the servcies will not start... serveradmin claims that IMAP is running but that SMTP/SMTPD/Virus Scanner/SPAM filtering/Virus updating are not. I tried disabling the all the filtering services but to no avail. Even though it claims IMAP is running I cannot connect from any client...
I have rebooted the machine but this did not help. I have looked at all the logs (system, mail, dovecot, postfix, amavis) and theer are no suspicious messages in any of them!
I am out of ideas and my mail server is down which is very serious so if anyone has any ideas please pitch in.
Thanks.

Answer 1

My problems were:
1.    Server.app shows the Mail service stuck in 'starting' status
2.    serveradmin shows that IMAP is running but SMTPD, Spam filtering, Virus filtering and Virus updating were STOPPED.
3.    No clients could connect to either IMAPS or SMTP.
Diagnostics performed:
All the various messafge logs (system.log, Mail.log, mail-info.log mail-err.log, amavis.log) show no abnormal messages.
DNS is working just fine.
In the end I did the following:
1.    Stop all mail related services.
2.    Make a safe copy of /Library/Server/Mail/Config
3.    Restore the previous days version of /Library/Server/Mail/Config from backup.
4.    Start the Mail service
And all was working again just fine.
I then did a file by file compare of all the configuration files between the workign and not working configurations and I found just the following two differences in postfix/main.cf:
Not working:
inet_interfaces = loopback-only
smtpd_helo_required = no
Working:
inet_interfaces = all
smtpd_helo_required = yes
I don't think the second difference is so critical but the first seems to break everything (not too surprising I guess). All the other files were identical.
So, the good news is that all is working again but the huge concern is why this change happenned, seemingly at random. My strong suspicion is that Server.app, at some point when I exited it havign made some minor change unrelated to mail decided to apply these incorrect changes to the Mail configuration. Why it would do this is beyond me. I think there are some subtle bugs in Server 3.0 (and maybe Server 2.2 as well)...