Thursday morning we’re going to replace the mail server cluster that handles delivery to customer mail spools. For all but a handful of users that are using procmail to filter their mail on our servers this upgrade is expected to be completely seamless. This upgrade will eliminate the queue delay problems that have shown up a few times over the last couple of months. For more information please see this forum post.
Update: The upgrades have been completed and everything appears to be working correctly at this time. Mail delivery latency is back under our target of less than 1 second.