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Vista/7/2008 can't telnet to a certain SMTP server external port 25, but XP can.

    Question

  • I have a really strange problem in my network.

    Any new Vista/7/Server 2008 can't telnet to a certain SMTP server external port 25, but if I reinstall it with XP it will do it. I've tried looking at the packets with Wired Shark,  it just won't connect.

    The problem began with Exchange 2010. We tested it and I found one message going out to this particular domain which has been working fine under Exchange 2003 and the company has been communicating with each other for a long time. The other company doesn't change anything on their end.

    Then we tried telnetting to port 25 from the Win2008 server it doesn't work.

    So long story short, I finally converted the old server from physical to virtual and this win2008 server is the host system. Guest what? The old Win2k3 server inside that Win2008 server can connect to that mail server with either Exchg or SMTP telnet to port 25!!! How bizzare is that? What's even more bizzare is I can connect to all other SMTP server on that Win2008 64-bit/Exchg2010 server. I'm quite sure this has nothing to do with Exchg 2010 since I can't even telnet to that particular SMTP server port 25.

    But the Win2008 server still can't do it.
    I've checked:
    1) Firewall config (and router firewall as well)
    2) Updated with the latest driver
    3) Disabled all kinds of blocking
    4) I can still connect to all other SMTP server
    5) Packet capture with Wired Shark, port 25... got two TCP DUP ACK when trying to connect to that server
    6) Cleared the ARP cache


    Now I have a test machine that came with Vista preinstalled. Then I tried telnetting to port 25 from that Vista and it can't do it! I tried updating with the latest driver - same!!! Then I downgraded to XP SP3 and I can telnet to that SMTP server!!! EHLO and everything else works just fine!! I also tried with an OS X machine and it can connect to that external SMTP server port 25 - in the same LAN!!

    *All* other servers running Win2003 in the same LAN can connect to that one external SMTP server (which is running Exchange 2003) and it is working fine!!

    I also tried connecting through a laptop with Windows 7 preinstalled and I cannot telnet to port 25 - same message "Could not open connection to the host, on port 25: Connect failed"

    So, here's my question:

    1) Why do WinXP and OS X work but not Vista/2008/Windows 7? Any change to the TCP/IP stack?
    2) How can I make the Win2008 work - telnet to that port 25?
    3) Why is the Virtual Machine (Win2003) inside the Win2008 can connect to port 25 but not the host machine itself (the Win2008) ?

     

    Tuesday, July 06, 2010 3:04 AM

Answers

  • Hello mrwaterdancer,



    This problem occurs because routers do not support the TCP autotuning settings.

    To work around this problem, disable autotuning.

    This article helps you work around this problem:

    How to disable the TCP autotuning diagnostic tool
    http://support.microsoft.com/kb/967475

    Best regards,
    Harry


    This posting is provided "AS IS" with no warranties, and confers no rights.
    • Proposed as answer by Harry Hi Wednesday, July 07, 2010 2:54 AM
    • Marked as answer by mrwaterdancer Thursday, August 05, 2010 8:07 PM
    Tuesday, July 06, 2010 11:18 AM

All replies

  • By the way, I've also ensured that antivirus isn't blocking anything. I've double & tripled checked the firewall rules. In fact, there's no antivirus on that Win2008 64-bit/Exchg 2010. There's also no antivirus running on all machines being tested. My own XP workstation, which has Symantec Endpoint Protection AV - can connect just fine.

    I was wondering if this has anything to do with TCP checksum offloading in Win2008 - but then I disabled it and it still won't connect to that particular SMTP server.

    Tuesday, July 06, 2010 3:07 AM
  • Hello mrwaterdancer,



    This problem occurs because routers do not support the TCP autotuning settings.

    To work around this problem, disable autotuning.

    This article helps you work around this problem:

    How to disable the TCP autotuning diagnostic tool
    http://support.microsoft.com/kb/967475

    Best regards,
    Harry


    This posting is provided "AS IS" with no warranties, and confers no rights.
    • Proposed as answer by Harry Hi Wednesday, July 07, 2010 2:54 AM
    • Marked as answer by mrwaterdancer Thursday, August 05, 2010 8:07 PM
    Tuesday, July 06, 2010 11:18 AM
  • OK I will try it now.
    Thursday, August 05, 2010 8:00 PM
  • WHOA IT WORKS!!!! YOU'RE AWESOME!!!
    Thursday, August 05, 2010 8:07 PM
  • I have a similar problem. I'm using Windows 7 and I cannot SMTP to port 25. I tried the solution posted above by Harry Hi, but with no luck. It may be a fix for Vista only.  mrwaterdancer, were you able to test this solution on the windows 7 configured machine? Here is a small log of my trial and error results.

    command line test
    telnet smtp.mail.yahoo.com 25
    telnet smtp.gmail.com 25
    Result -- Connect Failed

    smtp.icannotconnect.com
    attempt to establish a connection to port 25
    Result -- Port Blocked

    mxtoolbox.com
    Lookup smtp.mail.yahoo.com
    port scan result 25 and 587 are open

    Wormly.com
    tested SMTP server, but Authentication is required

    tried to disable autotuning
    http://support.microsoft.com/kb/967475
    No Go!!!

    Sunday, April 24, 2011 2:13 AM