For <mail2news@anon.lcs.mit.edu>:
To receive a list of all newsgroups send mail to
<mail2news@anon.lcs.mit.edu> with Subject "groups" (no quotes).

For <mail2news@mixmaster.shinn.net>:
Same method as above. Or you can finger <groups@mixmaster.shinn.net>
for a full listing of groups.

For <mail2news@dizum.com>:
It offers the same capability. Unfortunately, the last time I checked
the list of groups it sends back is incomplete and inacurate.  It's
safe to assume, however, that like the other two gateways Dizum
supports between 10,000-25,000 newsgroups from all the major
hierarchies.

You can also include an egrep-style regular expression on the subject
line.  For instance,

	Subject: list comp\.unix

would list only newsgroups whose names begin "comp.unix".

	Subject: list .*linux

would list all newsgroups whose names contain the substring "linux".

	Subject: list alt.*(security|privacy)

would list all newsgroups beginning "alt" and containing either the
word "security" or the word "privacy".

	Subject: list .*\.test$

would list all newsgroups ending ".test".