I have subscribed to the amavis mailing list and posted my question about the file descriptor problems I have been encountering in Zimbra. One of the respondents asked about amavis-specific logs:

Do you have anything (error messages) logged by amavis? Possibly in a
log by itself if not in mail.log? If you start amavisd-new using
'amavisd debug' (after stopping amavisd) (or amavisd-new debug), what
version of Net::Server does that show you are running (it lists
versions of helper programs). I'm looking to see if you have ever
installed 0.91 or 0.92 from source or CPAN which would obscure the
Debian version(s). As you have found out, Net::Server 0.87 0.90 0.93
or 0.94 should work fine with 2.4.1.
The wiki pointed to /var/log/zimbra.log for amavis log messages, is there any other location for them? Is there a way to start amavis in debug mode from within Zimbra?