Hi. Check the MTA Trusted Networks values for Global and server settings. I kept having AV/AS stop - in Global settings, the trusted networks are listed on one line, separated by a space. On the server MTA tab, they are separated by a line break - this breaks salocal.cf on my install (zcs-7.1.1_GA_3196.UBUNTU10_64.20110527011124). The setting in Server MTA tab won't allow networks to be specified on a single line.
I modified salocal.cf to put the two networks on one line, restarted amavisd and haven't had a problem all day. Unless I edit the server settings in the backend - in which case, the line break config returns. Have posted as a bug:
https://bugzilla.zimbra.com/show_bug.cgi?id=60590