We're having an issue where we are no longer receiving external emails. (We have an Exchange mail system, with a Barracuda spam filter and Watchguard hardware firewall.) The problem is that mail appears to be getting through the Watchguard box, but it is not showing up on the Barracuda box. I've been on the phone with techs from both companies and they both pretty much point the finger at the other box.
To help narrow down the problem, I'm going to hook up the outgoing port of the Watchguard box to a hub and connect my laptop, which is running Wireshark. My question is: what packets will I see that will tell me whether mail is getting through the Wireshark box? Can anyone recommend what filter I should use, or at least tell me what types of packets (e.g. will they show up as SMTP?). Or maybe I should just look for traffic on port 25? Thanks in advance.