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why port 137 is needed to send to exchange2003

By wxcheng ·
Some of our clients complain that they cannot send emails to us. We find that our Windows Small Business Server 2003 drops their smtp connection quickly if the sending server does not have port 137 open.

It is irregular. Why is 137 NetBIOS needed? For email communications, smtp 25 should engough.

I used PortsLock1.56 (from to monitor a whole session of telnet to our mail server's smtp port 25 and I saw a few records of port 137 activity.

Further test shows that if the firewall (very common these days) blocks port 137, telnet to our server's smtp port will fail.

What's wrong with our server's setting? Can I disable NetBios over TCP/IP on the server and WINS service to fix this problem?

you can try the test with port 137 open/blocked

telnet 25

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