There are times when there are SMTP entries in the TCPWARE_SMTP queue in a pending state, what causes this and is there anything I can do about it.

An SMTP entry goes into a pending state when there are no SMTP execution queues available to process the entry. This could be caused by not having enough execution queues to handle the mail load on your system. You can increase the number of execution queue to 3 per cluster node in by setting the QUEUE-COUNT in MU CONFIG/MAIL. For example -

$ TCPWARE CONFIGURE/MAIL
  SET QUEUE-COUNT DEFAULT 3
  EXIT
$ @TCPWARE:START_SMTP

The queue-count defaults to 1.