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I am in charge of a small company's IT department, and am well versed in UNIX/Linux (which our server runs) but not so well versed in Windows (which many of our employees run). We have a website and a mail server hosted at http://www.hostmonster.com/. About a year ago, when the website was made and the server configured, I set up all employees' Outlook to access our mail server using IMAP and SMTP, and there have been no major problems until now.

One of our employees, running Outlook on Windows 7, suddenly reports that his mail will not send, although he can check mail normally. He swears that he "didn't touch anything", and that all he did was visit his parent's house for a week. The problem is not with his router as the mail fails to check at his public library. Upon starting a Windows Remote Assistance session, I was able to determine that while pings to our mail server went through with normal latency, telnet to the server on port 25 fails. The same telnet succeeds on my machine. Moreover, telnet to smtp.gmail.com and mail.comcast.net, as well as a hastily opened port 25 on my home network all fail.

Suspecting a firewall issue, I disabled Windows Firewall as well as Kapersky AV, to no avail. I am currently attempting to contact him again in order to try the suggestions in How to check if a port is blocked on Windows?, but I would like to ask for other ideas as to the source of this problem or any possible suggestions.

Thank you very much.

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My guess is that SMTP traffic is being blocked by the ISP (both the user's ISP and the library's ISP). If you know that telnet to the server from your network works then the way to verify the user problem is to telnet from his computer while it's connected to your network. –  joeqwerty Sep 14 '11 at 0:40

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Gmail uses ports 465 and 587 for Gmail (SSL and TLS encryption). Most service providers, and most institutions like the library, will block port 25 and only open it up for their business customers.

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You can in fact telnet to port 25 at smtp.gmail.com. –  joeqwerty Sep 14 '11 at 1:11
    
I didn't know that. Cheers. –  Alo Sep 14 '11 at 1:15
    
I didn't either until I tried it one time on a lark and discovered that you could. At the very least it's a valid outgoing SMTP test. –  joeqwerty Sep 14 '11 at 1:23
    
Thank you, that is the best explanation I have been able to find. Interesting that it happened so suddenly, though. Anyhow, the server also connects on port 26 for SMTP, so I've had him change his Outlook settings correspondingly. Thank you for the help. –  Actorclavilis Sep 14 '11 at 1:42
    
I actually work for an ISP and we do it as standard practice. It's one of the reasons business accounts cost more than residential for the same bandwidth. Plus we, like most, have policies against running mail servers on a residential plan and mail clients get blocked as a side effect. –  Alo Sep 14 '11 at 20:27

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