A number of senders are reporting that they’re getting unusual responses from AOL servers. The responses include:
421 mtain-dk10.r1000.mx.aol.com Service unavailable – try again later
554 mtain-dk03.r1000.mx.aol.com ESMTP not accepting connections
mtain-dk* are assigned reserved IP addresses. It looks like something broke inside AOL again, and lots of places are having trouble delivering mail there.