[Billmonk][1] definitely. It sends emails whenever a new bill is shared among users, you can shuffle debts when many people owe money to others in the same group. It supports receiving instructions via [SMS emails][2] (SMS sent to an email address), but I'm not sure if a regular email would work though.

**Update:** Billmonk regularly fails to work correctly (repeating bills haven't worked for months now) and I wouldn't recommend people to use it anymore until everything is fixed.

  [1]: https://www.billmonk.com/
  [2]: https://www.billmonk.com/about/sms