Auto send messages to users after they've been signed up for certain time periods.

I'm looking for an add-on that will automatically send an email or private forum message to users that have been registered on the forums for certain periods of time.

I'm running a Minecraft server and would like to promote them in-game after they've been signed up on the forums for 1 week, 1 month and 3 months etc by sending them a different password for each time period.

If anyone can help, would be appreciated, even if you name a price I'm happy to donate for this if it works.