Yesterday, I had interesting mail problem that I believe is still occurring today only on a much smaller scale. One on my servers crashed a couple times. I believe this is because the server was overloaded trying to process too many mail messages at once.

I ended up having to add the following line to on both of my mail servers:

smtp-amavis_destination_concurrency_limit = 2

I found this in the amavisd documentation for a postfix installation. I never went into the performance tweaks documented. I had about 200 messages queued up on my secondary server. Whenever, I tried to process them, my primary server got pounded. I decided to process the couple messages that looked legitimate, and delete all the others. So far, with the new settings mentioned above and deleting the pending unimportant messages in the secondary queue, things have been working fine. I guess I should RTFM a bit more on postfix and amavisd. Especially, since I am running these domains on older hardware.