A workaround that worked for me is to use an admin mail as sender in mail.send_mail_to_admins.
All other valid senders for mail.send_mail I tried were invalid for mail.send_mail_to_admins. -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To post to this group, send email to google-appengine@googlegroups.com. To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.