Some people try to test out an application, a site, a contact form and may use an email like email@example.com that would never be delivered by the local email delivery system and would be in the queue forever, or until it is deleted.
Here’s a trick to delete such emails from postfix. Similar commands may be available with other similar tools too.
# to see the mail queue
# to delete all the deferred emails
postsuper -d ALL deferred
# verify that queue is empty
# for more info, check out the *man* pages of these commands