How to make sendmail accept connections from localhost only

I need to secure the server by making sendmail-mta accept only local connections (from localhost), so that any external (potential spam) connections are denied.

I use Debian 7.0 currently.

Solution 1:

The following line in your m4 config generation file will cause sendmail to listen to port 25 only on 127.0.0.1:

DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl

Solution 2:

I decided to do it in another way. Instead of trying to tune the the sendmail-mta itself (which I did not succeed in) or recompiling it with the built-in options, I used a simple iptables rule:

iptables -A INPUT -i eth0 -p tcp --dport 25 -j DROP

This rule blocks all incoming connections on eth0 interface. The connections to the lo interface remain untouched. Of course, this is not a solution by means of the sendmail-mta, but it turned out to be much more simple to solve this particular problem this way.