On your update question:
If all you want is to limit mails/second and you know how many hosts are doing the sending, you might as well throttle each host to 1/hosts of the rate one host would be sending.
So 2 hosts would be sending at 1/2 of the rate one host sends. No communication necessary for that

if you really really want communication, then the database all the hosts are already accessing gives a convenient means to coordinate.
For example it could give out contingents of email adresses to each host that are put in by one coordinator process at the specified rate.

Perl threads could be used similary, one coordinator thread and many sub-threads. Counter question: Safe from what?