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Tech Tip (LISTSERV) – Issue 3 – 2005

Q: How can I configure the Microsoft® IIS SMTP Service (SMTPSVC) to deliver mail from LISTSERV?

Answer by Françoise Becker
VP of Software Engineering, L-Soft

LISTSERV® requires an SMTP server to deliver the mail messages that it generates. While many sites simply configure LISTSERV to forward its mail to their main corporate mail server, some sites prefer to separate their list mail delivery from their "regular" mail. This is done by setting up one or more dedicated SMTP servers solely for LISTSERV's use.

The Microsoft IIS SMTP Service (SMTPSVC) is a free SMTP server available on the Windows platforms. With careful configuration, the recent releases of IIS SMTP can work with LISTSERV to handle small to medium loads on a single server – as well as some larger loads (depending on load patterns and delivery needs). With additional servers, this configuration can be used for loads of any size.

The IIS SMTP service should not be used to process incoming LISTSERV mail. The LISTSERV SMTP Listener service (SMPTL) must be used to process incoming mail on port 25, and the IIS SMTP Service (configured to listen on a different port) will be used solely to deliver mail generated by LISTSERV.

The steps required for configuration are:

  1. Install the Microsoft IIS SMTP Service (if not already installed).
  2. Configure Microsoft IIS SMTP Service to work with LISTSERV.
  3. Configure LISTSERV to send its outbound mail to the Microsoft IIS SMTP Service.

Following are the minimum steps that must be followed, otherwise LISTSERV mail may not be processed correctly through IIS SMTP. Detailed instructions for these required steps are given in the L-Soft White Paper titled Using Microsoft IIS SMTP Service for LISTSERV Deliveries, which also includes recommended steps for achieving optimal results, as well as tuning recommendations.

  1. Install Microsoft IIS SMTP Service according to Microsoft's instructions.
  2. Installation Notes:

    • IIS automatically puts the SMTP queue subfolder in the same folder as the Web service's home directory subfolder. Therefore, the IIS Web service's home directory should be on a drive with sufficient disk space to accommodate the SMTP queues. The exact size requirements will vary depending on your mailing volumes.
    • If LISTSERV Maestro is or will be installed on this server, the IIS Web service should be changed to listen to a different port than 80.

  3. Configure the SMTP Server using the IIS management application:

    1. Set the incoming TCP Port, to something other than 25 (for example, 50025).

      How: Properties -> General tab -> Advanced... -> select IP address -> Edit... -> TCP Port -> enter new value, e.g. 50025

    2. Set the "Recipients per message limit" to a value equal to or greater than the value of LISTSERV's MAXBSMTP site configuration parameter or change MAXBSMTP in LISTSERV's configuration.

      How: Properties -> Messages tab -> Limit number of recipients per message to -> check box, enter desired value, e.g. 1000 or 100

    3. Disable outbound delay notification. Set it to 9999 days to guarantee that the messages will expire before a delay notification is sent.

      How: Properties -> Delivery tab -> Delay notification (in "Outbound" frame) -> enter "9999" and select "Days"

    4. If IIS SMTP is on the same server as LISTSERV, configure SMTP to send bounces back to LISTSERV rather than treat them as "local" mail.


      1. Domains -> right-click on local domain name -> Rename -> change name (for example, "smtp")
      2. Action -> New -> Domain… -> Remote -> Next -> enter LISTSERV domain name -> Finish

  4. Define the following parameters in the LISTSERV site configuration file (site.cfg):
  5. SMTP_FORWARD=smtp-domain:tcp-port-from-2a

    If SMTPSVC is installed on the same server as LISTSERV, smtp-domain is the fully-qualified LISTSERV domain name (not the renamed name from step 2.d.i).

    For example:


Microsoft, Encarta, MSN, and Windows are either registered trademarks or trademarks of Microsoft Corporation in the United States and/or other countries.

Tip: High-volume sites require a certain amount of Internet mail expertise to get the tuning "just right". L-Soft offers comprehensive consulting services, providing your organization with in-depth customized assistance to get the tuning right for your site. See for more information.

Copyright 2005 L-Soft