What would be the least CPU intensive way of blocking mail to all domains except one on a cPanel server with Exim? I have a server that is getting spammed very hard and I don't want to have to modify MX records for all of the domains except one (there are WAYYY too many). My customer only needs email on one of the domains.. any ideas?

The *best* way is to do it via MX records. use a quick script to go through and do it for you. Remove the MX record from all zones via script, then manually add the MX for the proper zone. Now restart named.

Should take upward of 12.4332 seconds.