sendmail: implementing a “catch all” address

You may find yourself in a situation where you may need to implement a “catch all” email address, i.e. every email that is directed to your domain regardless whether the user exits or not, not to be rejected but instead directed to a single mailbox. There are various approaches to the problem, and we will see some here:

First, the easy way:

Using FEATURE(`virtusertable’) one can do that in a single line:

@example.com          catch-all@delivery.host.name

You can even exclude some addresses and have email delivered to their own mailbox instead of catch-all:

user1@example.com          user1@delivery.host.name
user2@example.com          user2@delivery.host.name
@example.com                 catch-all@delivery.host.name

The sendmail.mc way:

Normally the above trick which is adequately described in cf/README and the bat book, should be enough. But there may be cases that it is not the solution that you want, or simply because it-is-not-invented-here. For example you may want to redirect to catch-all all email directed to existing users of the system, as opposed to the virtusertable trick which does this unconditionally:

LOCAL_CONFIG
Kuser user -m -a.FOUND

LOCAL_RULE_0
R$- < @ $=w . > $*        $: $(user $1 $) < @ $2 . > $3
R$- . FOUND < @ $=w . > $*          $@ catch-all < @ $2 . > $3

Or, you may want to redirect to the catch-all address all email directed to non-existing users of the system:

MODIFY_MAILER_FLAGS(`LOCAL', `-w')dnl
FEATURE(`local_procmail')dnl
MAILER(`smtp')dnl

LOCAL_CONFIG
Kuser user -m -a.FOUND

LOCAL_RULE_0
R$- < @ $=w . > $*        $(user $1 $)
R$- . FOUND          $#local $: $1
R$-                    $#local $: bit-bucket

In fact (with bit-bucket aliased to /dev/null) the above example silently discards every email not directed to an existing user.