Ok, I'm about to lose my mind trying to troubleshoot this. Hopefully, someone can help me.
Dealing with an organization that has about 20-30 users. We're using POP accounts for e-mail (which we're hosting with MediaTemple). User workstations are mostly Windows 7 Professional. Outlook clients are mostly Outlook 2010, with I think a couple of them running Outlook 2007. For the last few months, we've had an on/off problem where our outgoing emails will flat-out stop working. Messages sent from Outlook will just sit in the outbox forever (for all users). We can still receive email when this is happening, but we can't send. Here are a few other bits of information:
- Sending works just fine through the webmail option we have with MediaTemple.
- Mobile devices work fine as long as they're not connected to the wireless network at the office.
- I can configure a POP account with Outlook 2010 on a laptop, entering a user's email credentials. Sending doesn't work when I'm at the office. It does work if I take the laptop home with me. Those emails that never leave the outbox? Go home and do a send/receive from there, and they all clear out. So....without changing the Outlook configuration on the laptop, it works from my house but not at this office. Which tells me that I have Outlook set up correctly and that it's not doing anything screwy. Something specific to that site is what's messing us up.
- Other POP accounts (I set up a couple of Yahoo/Gmail ones for testing) do the same thing, meaning I can receive but can't send from those accounts (unless I either use webmail or carry the laptop with Outlook off site).
- Multiple ports (25 and 587) do the same thing.
- mxtoolbox.com is giving me green lights on everything regarding my domain/SMTP functionality. No blacklists, no DNS problems, etc, according to it. The only thing that's not green on mxtoolbox is a timeout on Spamhaus ZEN (and the Spamhaus Project website confirms that our domain is not listed with them if I go there to check manually).
- We're running a SonicWall TZ205 with all the security services enabled. I have tried temporarily disabling all the security services, which didn't help. I also even tried temporarily installing a cheap "Wal Mart special" router that has pretty much nothing for security services, and that didn't make a difference.
- MediaTemple support says they're not doing anything to block us. Considering the fact that I can send from that laptop offsite with Outlook and through their webmail even at the office, I believe them.
- We have a block of static IPs with our ISP. If I change the WAN port configuration on our router to a different static IP, outgoing email starts working again. Then this problem reoccurs at some later point (first time it fixed the problem for about a week, this last time it fixed the problem for about 1-2 months). Stopped working today. I'm going to have to go back over there in the morning and go to the next IP up...
- No obvious signs of malware (my first thought was possibly someone on the network had a mass mailer that was causing our ISP to block us, but again, no obvious signs and nothing from any of my AV software, and again, our ISP denies any such blocking).
- ISP swears up and down they're not doing anything. They recommend changing my port number in Outlook SMTP settings to 587, which doesn't help. I've also tried even changing to port 465 (SSL) in Outlook, with no luck. But yeah, beyond that, conversations with the ISP's support are just becoming a "nope, it's not us" roadblock.
- Telnet testing doesn't work - I can't connect to anything on port 25/587 from that site.
I'm running out of ideas and would really like a permanent fix. I'm really thinking my ISP is doing something strange and that maybe I need to push them a little harder to figure something out, but it'd be nice to see if anyone else has any thoughts on something I could've possibly overlooked.