View postfix queue:

# mailq

View message header and content:

# postcat -q <ID>

Delete specific mesage from queue:

# postsuper -d <ID>

Delete all mails from queue:

# postsuper -d ALL

Delete all mails in the defered queue:

# postsuper -d ALL deferred

Requeue all mails:

# postsuper -r ALL

Requeue all deferred emails:

# postsuper -r ALL deferred

a few more hints over here