This is answered in the FAQ:  http://www.lsoft.com/manuals/lsv-faq.stm#2.12

Briefly, someone tried to send a mail-merge message, and you aren't running
LSMTP Classic as your outbound mailer program, so the proprietary SMTP
extensions used for LISTSERV's mail-merge feature aren't supported.

Nathan

At 11:27 AM 7/23/2003 -0400, Dave wrote:
>Some users have reported they haven't been receiving LISTSERV messages,
>despite the fact that I have run several successful tests.
>
>I looked into the issue and found that there is an infinite loop going
>on somewhere. The SMTPS log file repeats the following 3 commands every
>second:
>
>23 Jul 2003 00:00:01 *** LSMTP extensions activated ***
>
>23 Jul 2003 00:00:01 -> XDFN="John Smith"
>
>23 Jul 2003 00:00:01 500 Syntax error, command unrecognized
>
>This has apparently been going on for months.
>
>It seems that someone might have sent a DISTRIBUTE job with a syntax
>error. Still, I don't see why it should lead to this behavior.
>
>I've searched the spool directories and took a look at LSMTP but can't
>find any trace of this problem, nor can I figure out how to stop it. I
>don't know if it's related to the lost mail, but it's definitely a
>problem. Any help would be appreciated.
>
>--
>Dave Rubin