No, but if you do a SEARCH of the list's archives you will find that each
posting does have a unique number.  You can refer people to posting number
xxxx and tell them they can get it with the "GETPOST listname xxxx" command
(sent to LISTSERV@your host).

Note however that if you delete archives or physically edit the archive
files, these numbers may not always remain the same.  I would say it might
be better to refer people to postings by subject and/or author, then you
can do things like

// SEARCH * IN listname WHERE (SUBJECT CONTAINS subject-string) AND ,
   (SENDER CONTAINS author-string)

You can also do this kind of search from the web archive interface.

On Thu, 8 May 1997 22:11:10 +0900 FIRST NEWS newsmaster said:
>Hello.
>
>Is it possible to put sequential number on subject ?
>like...
>
>Subject: [samplelist 0001] sample title...
>Subject: [samplelist 0002] Re: sample title...
>Subject: [samplelist 0003] sample title2...
>
>We want to use these numbers to refer prior postings.
>
>Or is this not common practice with LISTSERV ?
>
>Thank you for your help.
>
>Takashi
>from Tokyo Japan
>
>
>
>--
>Takashi Hakii - FIRST NEWS newsmaster -
>FIRST NEWS is FREE weekly URL newsletter in Japanese.
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>TRICORN Corporation Tokyo Japan