Windows, unix


The Internet hostname of the server to which all outgoing SMTP mail should be forwarded for delivery.



<not available>







The SMTP_FORWARD host can be any machine with SMTP MTA software that will accept mail from your machine.

By preference, this variable should contain a fully-qualified domain name [FQDN]. Alternately it may contain a dotted-decimal IPv4 IP address, but this is not recommended.

Unqualified hostnames are not supported and will result in errors.  If you intend to forward mail to the local host, again by preference, we recommend using the FQDN of the local host, rather than "LOCALHOST".

IPv6 addresses are not supported.

Implementation Notes:  SMTP_FORWARD is a synchronous mail handling method, which means that it is part of the main LISTSERV process, and will be invoked by the main process for every piece of mail that LISTSERV sends.  While this can be very efficient if the SMTP_FORWARD host is properly configured to accept LISTSERV mail, synchronous mail handling has the potential to slow LISTSERV down considerably when very large lists or very large mailpieces are encountered.

Very early after the "workstation" operating system ports were created in the 1990's, this problem was recognized and an asynchronous handoff method was implemented, called "SMTP 'workers'".  These "workers" are defined by the SMTP_FORWARD_n= site configuration variable, and importantly, LISTSERV can be configured to spawn multiple "workers" for very high-efficiency handoff to your campus or third-party SMTP MTA.  When SMTP "workers" are defined, LISTSERV simply writes .MAIL files into its SPOOL directory instead of trying to deliver them directly, and they then are picked up and handled in a first-come first-served manner by the waiting "worker" processes.

Today, the original SMTP_FORWARD synchronous method still has an important part to play -- it acts as a fallback if for some reason asynchronous delivery fails -- and it should always be defined to point to a working SMTP MTA for that purpose.

Default Value


The default is to point to the localhost; however, it is strongly recommended that this parameter be set explicitly with the FQDN of the host.


None. This parameter must be set explicitly.


Not allowed.

See also