Hi tlhIngan
If your server is using exim (typical on cPanel servers) here are some examples of command line mail queue management.
Display how many messages are in the queue: exim -bpc
Print a listing of the messages in the queue (time queued, size, message-id, sender, recipient): exim -bp
Print a summary of messages in the queue (count, volume, oldest, newest, domain, and totals): exim -bp | exiqsumm
Search the queue for messages from a specific sender: exiqgrep -f [user]@domain