Postal is used to benchmark SMTP servers.
I am using it so that I can send mails continously to my test server and hence check if it is working properly. Hence this saves me from the pain of composing mails everytime I do some changes in the server and check if the changes I made is what I intended.
All I have to do is execute one command given below.
postal -m 100 -f test -t 5 192.168.31.100 users_mail
users_mail has the list of all the users in my test server.