If yours is an upgraded Mountain Lion machine, as opposed to one that has been clean installed, you may see this problem in the log :

Aug  6 07:57:41 iMac.local postfix/postmap[2571]: fatal: bad string length 0 < 1: mydomain_fallback = 
Aug  6 07:56:54 iMac.local postfix/postqueue[2537]: fatal: chdir /Library/Server/Mail/Data/spool: No such file or directory

A clean-installed machine is OK - it doesn't exhibit this problem.

If yours was an upgraded Mountain Lion machine, the problem is that Apple assumes that it is doing a Mountain Lion Server upgrade and gives it the "wrong" main.cf file (meant for Mountain Lion Server).

I have a new version of MailServe, 6.0.2, that attempts to fix what Apple has wrought.

It attempts to give you a main.cf file in /etc/postfix that is meant for Mountain Lion client. I don't use /etc/postfix (I have my own installation directory in /usr/local/cutedge/postfix) but built-in commands like sendmail, PHP mail, and the mail command line still use the postfix config file in /etc/postfix, and that's the one emitting all these messages.

To fix this, run MailServe 6.0.2 and do a de-install from the Help menu. You must first make sure that Postfix and Dovecot are not running. Option-Click on their respective buttons to stop them. (Note :You may want save the current config from the File menu first). I patch the faulty /etc/postfix/main.cf in the de-install process.

After starting postfix again in MailServe, Mail.app should work with your server, plus all the other command line tools.