Exim is by default mail server in WHM/Cpanel, the mail use Exim to controlled all the task of mailing server.
There is some of command to perform task on exim server.
1. exim -bpc to check no of mail in queue
2. exim -bp to check the no of mails in queue and also their detail.
3. exim -qff -v forcefully send mail from queue.