The smtp-source program is an SMTP benchmark and load testing program included with Postfix. It has a flexible set of options for testing SMTP servers.

An example of using smtp-source:
smtp-source -c -l 100 -m 200 -f This e-mail address is being protected from spam bots, you need JavaScript enabled to view it -t This e-mail address is being protected from spam bots, you need JavaScript enabled to view it mail.foo.com

Here is what the options mean:
-c = show count of sent messages
-l nnnn = length of each message (uses X's for body)
-m = number of messages to send
-f = MAIL FROM
-t = RCPT TO
the last option is the receiving mail server

By timing how many messages a server can receive, you can get an idea of the load it can handle.