sendmail/saslauthd Domain Blocking

[Tim Daneliuk]

I block unwanted domains from sending mail to one of our servers by
putting it on the reject list in /etc/mail/access.

I am seeing distributed brute force attempts to use that same
server as a relay.  These are coming from a few domains.
So far, these attempts have failed but I'd like to be proactive in
preventing future such intrusions.

Is there an equivalent way to block entire domains and/or subdomains
from ever even connecting to saslauthd?