Those of you using a SonicWALL TZ170 with intrusion Prevention Service may be having issues receiving mail right now. Seems that SonicWALL released a signature at 13:39 today that is trying to combat the Exchange security flaw. Well they fixed it. They managed to shutdown all smtp traffic. You will see an error like this in your log file.
IPS Prevention Alert: SMTP Exchange Meeting Request Attempt, SID: 3233, Priority: Medium
Normally you can just disable this one alert and things would be fine, but it seems that you have to disable IPS completely to get mail flowing.
I'm on hold with SonicWALL right now..........