Postfix
From Futuragora Wiki
Postfix is a mail server, just enter the following command to flush the mail queue:
postfix -f
Other cleaning mails commands:
postsuper -d ALL Delete differed mail queue messages
postqueue -f Process queue. Or: postfix flush
postcat -vq XXXXXXXXXX > themessage.txt save mail to file
mailq display queue or: postqueue -p
postsuper -d deferred mail clear queue