A couple customers have written in this morning to report issues with SQMail. I’m looking at this now.
UPDATE @10:25am ET – I found the cause – one of GeekISP’s firewalls rebooted last night, and the daemon startup sequence was such that haproxy, which is used to handle load balancing of some internal traffic, started after the dns cache. That caused haproxy to be unhappy and thus not forward connections to the backend mail server, though it was perfectly happy to accept and then close them immediately! Thus the empty server reply error message.