Ok, see things clearer now. The error with your mail server is that it tries to pass the mail to the filter and it fails, possibly it means that is a error in the filter configuration.
To try to check what is this error we could try to run the filter server manually and see what is the exact problem.
To try this:
enable mailfilter set the mail module to not use any filter to not disrupt your email
To be sure that the fitler is stopped run as toot 'stop ebox.amavisd-new'
Then use this command and check the output for errors:
/usr/sbin/amavisd-new -c /etc/amavis/conf.d/amavisd.conf foreground