My postfix server is not delivering emails to the users Maildir and the messages are staying in the mailq. I have Postfix, dovecot, Mailscanner, spamassin and clamav installed.

I have these errors listed in my system log:

Mar 22 15:09:43 server1 su(pam_unix)[6426]: session opened for user root by (uid=0)
Mar 22 15:09:43 server1 su(pam_unix)[6426]: session closed for user root
Mar 22 15:09:51 server1 postfix: Process did not exit cleanly, returned 255 with signal 0
Mar 22 15:10:24 server1 last message repeated 3 times
Mar 22 15:11:30 server1 last message repeated 6 times

Can anyone tell me what I can do about this.

Postfix seems to be calling a different process to deliver the mail, and this process failed with an error.
Postfix then requeues the mail to try again later, assuming that mail delivery failed.

My suggestion: try to find out what program is called by postfix to deliver the mail.
The transport configuration (main.cf and transport.cf files) should help you with this.