D.2.4.2 Using "EXPN" to Expand an Alias or List

The "EXPN" command allows you to "expand" a mailing list or an alias, to see the accounts underneath it. For example, if you have an alias "postmaster" account that is re-routed to another account, then you could "EXPN postmaster" to see the destination mail account(s) for the "postmaster" alias.

The "EXPN" command is often used with mailing lists, although it can also be useful for debugging purposes. If mail being sent to a particular list or alias appears to be getting lost, you can Telnet to the SMTP server on the remote system and "expand" that list or alias, as illustrated in Figure D.6 below.

Figure D.6 Using "EXPN" to expand an alias.

In the example shown in Figure D.6 above, an alias for "postmaster" refers to the "supervisor" account.

Note that you may not wish to enable the "EXPN" command on your SMTP server, as it can be considered a security risk by some organizations. Although it cannot be used to cause damage to your systems directly, it can be used to provide hackers and mass mailers with the ability to locate user accounts. Once they have determined a user account they can attempt to determine a password for that account. For information on how to disable the "EXPN" command, refer to section 6.4.1 Disabling the EXPN and VRFY Commands.

For information on how to use the SMTP "VRFY" command, refer to section D.2.4.1 Using "VRFY" to Verify a User. For more information about other SMTP commands, refer to section D.2.1.1 Common SMTP Commands. For more information about the SMTP response codes, refer to section D.2.1.2 Common SMTP Response Codes. For information on how to isolate out- bound delivery problems, refer to section H.6 Debugging Mail Delivery Problems.

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