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SMTP, POP3 and Telnet in Exchange 2000/2003

How can I verify that my Exchange server is accepting connections? How can I connect to my Exchange server via Telnet?

Sometimes you might want to troubleshoot your Exchange server from outside your corporate LAN (or from within, it makes no difference) in order to see if it accepts incoming connections. An example for this situation might be when your users are complaining about lack of incoming mail or lack of ability to pull mail from the server.

Using PORTQRY.EXE

A quick way to check SMTP connectivity is to use the PORTQRY.EXE command (see Quickly Find Remote Open Ports page for more info):

Using Telnet

You will also need to check various settings for possible problems, such as your virtual servers status, connectivity to the Internet, presence of a valid MX record and authentication problems, but if all seems to work fine and you still want to check the connectivity yourself – you can try it via Telnet.

Telnet forms part of the basic TCP/IP protocol suite and is therefore to be found on most machines that have TCP/IP installed. It is a very useful diagnostic tool that we can use to establish a connection to a remote computer (via a particular ‘port’) issue commands to the remote computer, and see the responses it produces.

The syntax used to run the telnet program is

where computer is the hostname, or IP address of the remote computer, and port is the TCP/IP port number that the service we are using runs on (25 for SMTP, 110 for POP3).

Sometimes you’ll want to see what you type in the Telnet console, so you’ll first start Telnet without actually connecting to the server, type

(or set localecho in Windows XP)

and then open the connection by typing

open <computer> <port>

SMTP and Telnet

SMTP is the TCP/IP protocol used to send e-mails via the Internet. It uses port number 25, so in order to use telnet to open an SMTP session with an Exchange server we use the command

If the connection is successful, we should see a banner and a command line interface. We then need to follow RFC 821 for the accepted SMTP commands, which are quite simple (as the name says…)

(Screenshot of the same dialog)

If the server accepted the e-mail, and if the recipient got it ok, there are no connectivity problems at your server and no firewall or stopped services issues.

POP3 and Telnet

POP3 is the TCP/IP protocol that is used to retrieve Internet mail from a server. It uses port number 110, so to open a POP session with our Exchange server we use the command

You will then begin to authenticate to the server and issuing POP3 commands as described in RFC 1939:

(Screenshot of the same dialog)

As with SMTP, if the server accepted the connection, and if you were able to retrieve your e-mail, there are no connectivity problems at your server and no firewall or stopped services issues.

Related articles

You might also want to read the following related articles:

Links

RFC 821 – Simple Mail Transfer Protocol

RFC 1939 – Post Office Protocol Version 3

Related Topics:

  • Exchange Server
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