If you would like to test the mail performance of your server then you could use jetstress to test your database. But when you would like to decide yourself what kind of e-mails you want to send to your mailserver then you should have a look at Blat. Blat is a command-line mailer which can be used in a batch file which send multiple mails to your mailserver. This program is not ment to be used for bothering other people with bulkmails. If you are interested in the tool then have a look at the site below.