Add-on for sending a sequence of emails

Is there a simple gem / engine / app for sending a sequence of emails - something like iContact or ConstantContact.

For instance, I want to send an email when someone registers on my site, then 3 days later, then 10 days later. That’s a rather simple case - would be nice to have some more complex workflow but this would do for now.

This would not be too difficult to implement, but issue of formatting, timing, error handling, etc I imagine may have been solved nicely by someone. Hopefully a gem exists - I have not found one yet.