No Free Workers Error
If you have not already, you should install PgBouncer as per Appendix G in the Administration Guide. This can greatly increase database performance.
You can increase the maximum number of MIMEDefang processes by editing /etc/mail/canit/canit.conf and adjusting the:
setting in the [mimedefang] section) of the /etc/mail/canit/canit.conf file. Make sure that you don't increase the maximum too much; each process uses about 50MB of memory. Also, the sum of mx_maximum values cluster-wide should not exceed the max_connections PostgreSQL setting (if you are not using PgBouncer) or should not exceed it by a factor of more than 5 (if you are using PgBouncer).