It seems all the measures we put in place yesterday weren't enough. We've just been paged by our servers that an inbound delayed mail queue has built up again. We're seeing what we can do bring it back down.
Update (2:30pm EST): The queues are going down. We're getting NYI to install 2 new servers ASAP that we previously were going to use for IMAP expansion and try and bring those up to help with the processing.
Update (3:05pm EST): Over half the mail from the queues is now delivered. Any new mail arriving should be delivered within about 1-2 minutes, old mail is being delivered as the queue can be cleared.
Update (4:05pm EST): Almost all of the queues have cleared now and all new mail should be delivered immediately.