Quoting Simon Hobson (linux@???):

> Perhaps I'm missing something, but doesn't SRS provide a gaping wide
> chasm for spammers to pile through ?

I would call _gaping_ chasm an exaggeration -- but it is certainly
abusable (to the extent cross-domain aliases become known or
discoverable in public).

Someone trying to send Don Marti spam via alias 'don@???'
will implicitly rope my linuxmafia.com MTA (mail transfer agent = SMTP
daemon) into the evil task of pumping spam delivery attempts at Don's
zgp.org MTA, a regrettable case of 'Let's you and him fight' -- which is
why I've just now permanently disabled (now that I remembered the
problem) all cross-domain /etc/aliases entries. (I've retained
intradomain aliases, such as ones that send root@, postmaster@, abuse@,
and hostmaster@ to the appropriate user mail spool.)

Relevant to this picture is the _other_ difference between MLMs (mailing
list managers) and other SMTP mail reflectors: MLMs are _smarter_,
giving opportunities to reject or sequester abusive mail patterns the
other reflector types cannot. E.g., even by default, GNU Mailman will
intercept and hold or reject mail with too many recipients, overly large
mail, mail implicitly addressed (mailing list address specified only in
Bcc), and a number of other similar heuristics. Also, in this decade,
almost no mailing lists pass through without review mail from
non-subscribed addresses -- and spammers have still shied away from
making their spambots go through 3-way confirmation to join mailing
lists before trying to post to them.

So, yeah, you have a point, and I thank you for the reminder that I was
long overdue to disable those aliases.