Our SBS 2008 server with Exchange 2007 started rejecting some (most) emails starting about 3 or 4 days ago. The return failure message looks something like this:
The error that the other server returned was: 451 451 4.7.0 Timeout waiting for client input (state 18)
I've rebooted our Sonicwall TZ-210 router and that doesn't make any difference. I think this problem has something to do with MTU (packet size) but I don't understand why the problem just started now. We haven't made any changes to our network infrastructure for weeks.
If I ping our SonicWall (DSL connection, bridge mode) forcing unfragmented packets, I start getting packet dropouts at packet size 1320 or so. Maybe this is completely normal, I'm not sure.
I haven't tried rebooting the DSL modem yet because I'm not on site. That's one of the next things I'll try.