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Problems with Exchange are sometimes due to it not responding correctly to LDAP or SMTP queries, possibly because another service (a firewall or a rogue application) is blocking access to LDAP or SMTP. As a result, even though Exchange may be running perfectly in some ways, it may not respond to certain queries at all -- and it won't even write any errors to the system log that you could use to diagnose the issue.
One common way to determine if Exchange is listening correctly is to open a telnet session to the port and machine in question, but the amount and variety of information you can glean is limited. A program like PORTQRY is better.
PORTQRY, or Port Query, is a tool written and offered by Microsoft to diagnose and analyze problems with servers that are not responding correctly to queries on specific network ports.
PORTQRY can scan and return extended information on ports used by many common Exchange services: SMTP, POP3, IMAP, LDAP, RPC, NetBIOS and DNS. The program can return three basic states for a given port: listening, not listening and filtered. Listening means a service is waiting for requests on that port; not listening means no service is waiting; and filtered means a service is listening but not responding.
PORTQRY uses the list of ports detailed in the %SystemRoot%\system32\drivers\etc\services file to determine what sort of packets to use (i.e., TCP versus UDP) and what type of service might be listening. It can also return extended LDAP query information on port 389, and can query port 135 to learn which programs on the target server are registered with its local RPC end-point mapper database (a useful way to determine if another service is interfering with incoming data).
PORTQRY can be downloaded here.
A graphical interface called PortQuertyUI is also available that makes using PORTQRY much easier, especially when probing multiple machines.
About the author: Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter and a regular contributor to SearchExchange.com.
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