>I keep getting junk mail in my listserv from automatically generated
>e-mail accounts that do not exist.  How can I prevent those to come
>through this listserv?

I use a CONTENT_FILTER, but mostly to block unwanted types of mail from
subscribers. (Out-of-office replies are an example.)

If you are using a later version of LISTSERV (14.3 or newer), junk email
from accounts that "do not exist" should be a good reason to consider
  Send= Public,Confirm,Non-Member

I assume you're already using Public, or this mail wouldn't be getting
through. Adding Confirm,Non-Member requires all posts from nonsubscribers to
be confirmed. LISTSERV emails the request for confirmation, and if the
source is a spammer fake address, nobody sees it, so the message is blocked.

Hal Keen