I'm trying to troubleshoot a problem with outgoing email that popped up this morning, and I am running out of ideas. About 7:30 this morning, Internet access for the site went down. We called our ISP, they said they were having an issue, and they sent a tech on site who fixed it. However, since then, our email (outgoing) hasn't been working. First, a little information about the site and how things are set up:
Office with about 30 users, all running Outlook clients (versions vary from 2007 on a few to 2013 on most). We have a SonicWall TZ205W appliance with Comprehensive Gateway Security Suite enabled behind the ISP's router. We don't host our email on site (it's run through our web hosting provider). We are sending on port 587 (although I have tried switching around the settings for one of the users like using port 25, trying 465 TLS, etc, all with the same results).
Users can send/receive email just fine if they send from the "webmail" option through the hosting company. Users who have laptops can also send/receive from Outlook just fine, as long as they carry those laptops off site. One guy was unable to send/receive from his desk at the office with Outlook, but the messages he had stuck in his outbox all went out once he took the laptop home. This tells me something on site is wrong.
The ISP swears up and down it's nothing to do with them, and the hosting company swears up and down it's nothing to do with them. According to mxtoolbox.com, the domain is not on any blacklists and passes all SMTP tests they have.
From on site, I can telnet to smtpout.secureserver.net on port 25 and send if I authenticate with my GoDaddy account, but I can't do that from a POP client on any workstation.
I've done a little testing on site with my laptop (plugged into the 24-port switch behind the SonicWall) and Mozilla Thunderbird, which has a couple of accounts configured.
GoDaddy account - (using 587 for outgoing) I can receive but can't send.
Gmail account - (using 465/TLS for outgoing) I can both send and receive.
Interestingly enough...when I tried to set up a POP account for one of the users here in Thunderbird, I get an error that the mail server settings couldn't be found, and it refuses to even set up the account. But if I carry my laptop offsite, I can set up the account in Thunderbird and send/receive from it just fine.
So...I am at a bit of a loss. The SonicWall isn't showing anything particularly strange. We run AVG antivirus, and AVG hasn't reporting anything strange. Also: plugging directly into the ISP's router (bypassing the SonicWall) with my laptop yields the same results described above.