The confirmation mail for commitments has an invalid envelope sender set. This causes it to fail sender callout verifications. If you do not want to receive bounces, just use an empty envelope sender.
2010-06-17 13:26:53 H=mail1.stackoverflow.com [126.96.36.199] sender verify fail for <email@example.com>: response to "RCPT TO:<firstname.lastname@example.org>" from aspmx.l.google.com [188.8.131.52] was: 550-5.1.1 The email account that you tried to reach does not exist. Please try\n550-5.1.1 double-checking the recipient's email address for typos or\n550-5.1.1 unnecessary spaces. Learn more at\n550 5.1.1 http://mail.google.com/support/bin/answer.py?answer=6596 25si11950938mul.54 2010-06-17 13:26:53 H=mail1.stackoverflow.com [184.108.40.206] F=<email@example.com> rejected RCPT <firstname.lastname@example.org>: Sender verify failed