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.
Best Answer
After reading up more online I did some ping tests using the -f switch and found that this particular DSL connection started dropping packets that were in excess of 1300 bytes. Microsoft refers to this as a "black hole router". Basically, something is wrong with that port.
I implemented a work around for this problem by going into the PPoE connection settings inside the SonicWall TZ-210 and changing the MTU to be somewhere in the 1250 range (I don't remember the exact number). Incoming emails started to flow again immediately as well as any other traffic that uses that particular connection. I'll have to contact SonicWall to see if we can have this problem covered under warranty.
Update1
As it turns out, this was apparently a DSL modem problem, not a SonicWall problem. Changing the MTU setting in the PPoE settings on the router did prove to be a sufficient work-around until the modem gets replaced.
Update2
A new DSL modem did not resolve this issue. Still not sure which device or software is causing this problem.
Update3
I guess I failed to post back here as to what was really wrong. The phone company found some bad hardware in one of their boxes or offices and replaced it. If you're troubleshooting this issue the tricky part is to determine if it's you're own hardware or something outside your LAN.