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I have 3 computers in my network connected to the same router.

Computer A - my desktop computer Computer B - Web server Computer C - Mail server and SQL server

Everything is set up correctly externally which means I can telnet to IMAP port (143) on Computer C and Web server port (80) on Computer B.

But suddenly, in like a 10 minute interval or more often I cannot telnet to Computer C from the outside. I can telnet from Computer A and B. And I can telnet from Computer A to B from the outside.

I do not think the problem is the router. Computer C is new with Windows 2012 and I guess it is either a network problem, configuration etc on that computer. Do you have any more ideas of how to test this or?

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It is possible your ISP is blocking your mail server when it detects it. – Scott Goodgame Jun 29 '13 at 12:18
No, I have mail server on Server B. And it works with same ports. Thanks anyway! – Henrik Jun 29 '13 at 14:06
Got windows update recently? Some windows update may change your firewall rules... (on computer C). – Darius Jun 29 '13 at 16:07
Thanks for your feedback - found the reason - the switch got the same ip as the computer C. – Henrik Jul 1 '13 at 7:35

Found out the reason. The switch and the computer accidently got the same IP :(

Thank you all for your feedback!

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