Super mailer, dark mailer, desktop super server, send-safe, there are a few others but they are slipping my memory at the moment. These programs all use custom strings/macros to insert almost anything you want. For the date it would be something like

Subject would go here [$date] or [%date]

[%time] [%randnum] or you can have it pull from a text files as well so your message greeting, closing and even words in the message are always changed. If you have say 4 variations of your letter, after you randomize certain parts of them you end up with a shitload of different letter variations.

All of this is done in an effort to beat the spam traps, that is why you see people still trying to put asoiufaosiuf after the subject line or just at the bottom of the message. Problem is this methods worked about 4 years ago, but since then is just another flag the spam traps pick up on.