Sackville-West Clan Wiki/ tech/ SMTP data timeout

Since we moved to Oregon, we've been seeing lots of "SMTP data timeout" in the exim 4 logs. After a little googling, it turns out this is a common problem with residential DSL. Residential DSL generally uses PPPoE which uses 8 bytes out of a packet for its routing information. That means that if a server tries to send you a typical 1500 byte network packet, it will bounce. Most servers handle this properly by then probing for the proper packet size (1492 because of the PPPoE overhead) and resending. But not all servers do this. Some servers just keep trying to pound that 1500 byte packet into a 1492 byte pipe and failing. The result is that the mail server, having easily received the small SMTP handshake, is waiting around for a large packet from the other end that never arrives. Eventually it gives up. I'm not really sure what happens to the email on the other end, but I assume it eventually evaporates in a cloud of wayward bits. The solution is to munge the MTU size on my end so that my machines never report that they can receive a 1500 byte packet. Doing

ifconfig eth0 mtu 1492

has totally solved a problem. Down from many of these a day to none.