Secondary MX user list filter for Sendmail

[Todd Vierling <tv () duh org>]

A colleague asked me offlist about how to make a Sendmail secondary MX
properly return 550 for invalid recipient addresses.  For smaller sites --
or even larger sites if the list is autogenerated and the access_db uses
"btree" storage (it's rather fast) -- the below solution will do just that.

=====

In your secondary MX's sendmail.mc, add the following FEATUREs if you don't
already have them.

FEATURE(`access_db')dnl
FEATURE(`blacklist_recipients')dnl

Rebuild your sendmail.cf from this, and then put entries like the following
in your /etc/mail/access file (assuming "foo.com" is your domain).  I'm also
assuming that "foo.com" is already in /etc/mail/relay-domains (or
/etc/mail/sendmail.cR depending on your sendmail install), so that relayed
mail can get through.

userone () foo com                    RELAY
usertwo () foo com                    RELAY
To:foo.com                         "550 User unknown"

The RELAY lines designate explicitly allowed recipient addresses for
relaying to the primary MX.  The To:...550 line, which requires the
"blacklist_recipients" feature, is a fallback rule that matches after the
email addresses and returns the expected "User unknown" error.

Now rebuild your access.db file, restart sendmail, and voila, you have a
secondary MX explicit user list.

-- 
-- Todd Vierling <tv () duh org> <tv () pobox com>