If you are having severe performance problems, check if multiple reprocess jobs are being generated. imta process<ret> should show only a single reprocess running. If more are showing, look in /etc/opt/SUNWmail/imta/aliases, and verify that it contains the following line:
postmaster: <user-name>@FDQN
Make sure the postmaster address is valid and is receiving mail. If it doesn't, add this line do a fresh restart of the IMTA.
# imta restart