We had an issue that I became aware of around midnight, but that probably had been going on earlier in the day, where mail backed up in queue on our servers that take mail incoming from the Internet, rather than deliver properly.
There were multiple issues, Yahoo couldn’t handle list mail as fast as our server wanted to send it, so that caused some congestion of the mail queue.
I didn’t edit the network-config script when I changed name servers so the machines reverted to old name servers not as well tuned to handling the mail volume.
The lock file on the bayesian spam filtering didn’t get removed during an update for some reason. This also caused things to back up.
The number of processes postfix was allowed to fork was excessive and driving the machines into swap, that also has been adjusted.
It isn’t completely finished processing the queue but the servers are chunking away at a good rate and should be fully caught up in a couple of hours.