Our primary mail server, treehouse.forest.net, is getting hit by an unusually high number of directory harvest attacks today. This results in saturation of our inbound SMTP connections, preventing users from sending mail through our server.

Our spam protection service, Postini, is detecting and blocking these attacks as quickly as it can. As the attackers quickly shift to new IP addresses, however, we are experiencing surges of activity followed by periods of calm. Thus, until the attacks stop, we can expect intermittent connection problems to continue.

This may give you an idea what we're dealing with. You can see a number of attacks, followed by decreased activity when the IP address in question is blocked, over the course of an hour. It ends with a particularly strong attack that maxed out all 500 of our connections for over a minute.