There was a 12-hour outage to the mail system of the site today.
Luckily, I was notified by a few users that their emails were no longer going through and that even the confirmation emails were not being sent, in fact, no emails were being sent. I was able to get a hold of Google (who is hosting Sendrecurring) and get the issue resolved
The issue was due to Sendrecurring switching to the high replication datastore recently. The Appengine service gave Sendrecurring the “default” limit of 100 emails per/day after the switch, to be upped back to the regular limit after the first billing cycle (one week). Well, Sendrecurring hit that 100 limit very quickly before users started noticed the problem. The limit is back up to the normal one now.
Sorry for the gap in service.