Earlier today, our current upstream mail provider suddenly disabled our account without notice or explanation. As a result, outbound mail is no longer being delivered. We've followed up with them (at 11:06 AM PST) but haven't received a reply yet.
In the meantime, I'm looking at transitioning outbound mail to a different provider (say, one that doesn't terminate service for long-term paying customers without contacting them). We didn't anticipate our provider just turning us off without warning, so we didn't have a redundant provider ready. Amazon SES is the most promising alternative provider, but has default service limits which are too low to accommodate our current outbound volume. I submitted a support request to raise these limits (at 11:49 AM PST) but haven't received a reply yet.
So we're waiting for either provider to get back to us and give us an outbound pathway. Outbound mail is currently queued in Phabricator, and will all be delivered once an outbound pathway is available.
There are minor technical barriers to delivering via SES in the cluster. I'm working to remove these while we wait for a reply so we can transition to SES immediately if that pathway unblocks first.