Are you sure this is coming from LISTSERV?  This sounds like an error
coming from the MTA, not from LISTSERV itself.

On Tue, 18 Jan 2000 13:44:15 -0500 Jonathan Waggoner said:
>  I'm using a dynamic web server to build a mail message to one of my
>lists which adds or removes a user from the list. It just takes an
>"email" text field and puts it into the <FROM> field of a message. The
>body is then the proper command. Unfortunately, all email coming from
>this app bounces back from LISTSERV with the following error
>information:
>"550 5.7.1 relay restricted"
>
>  Is this somehow related to SPAM checking and the fact that the
><SENDER> and <FROM> address don't match? Can I set my site variables to
>accept these type of messages from a specific IP (my web server)?
>
>--
>Jonathan Waggoner
>Web Developer
>National Gardening Association
>www.garden.org