Last week one of our shared servers mail queu reported over 2000 emails in queue to be delivered to email@example.com. Searching google about it yield no result. I tracked down the origin of the emails to a customer’s wordpress blog and It turned out that
a nulled pirated plugin from codecanyon (possibly infected with malware) has been sending emails constantly to firstname.lastname@example.org.
I am not sure what was the purpose of the emails, if it was an attempt to flood the mailbox or a “call home” feature but disabling the plugin solved the issue for us.