This is a sort of non-perl answer, and it may be wholly inapplicable to you.

However, the result your're looking for (minimising nslookups in the MTA) is pretty much only achievable within the MTA itself. I suggest exim http://www.exim.org - it already understands the concept of caching answers and multiple emails destined to the same recipient MTA within the mail queue (and this is smarter than the "domain name" choice, it's based on the MX for that domain)

(p.s. nice name, yojimbo! )

Quick wafting zephyrs vex bold Jim

In reply to Re: Sorting a list by frequency of items by yojimbo-san
in thread Sorting a list by frequency of items by yojimbo

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