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 -
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.