As posted to my personal forums
(forums.gfreeman.com):
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User-A ----> SERVER-A ----> (internet) ----> SERVER-B ----> User-B

User-A connects to SERVER-A to send an email to User-B
SERVER-A sends email to SERVER-B
SERVER-B send copy of email back to SERVER-A asking for confirmation
SERVER-A replies to SERVER-B saying "yes User-A sent that message"
SERVER-B passes email to User-B
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Yes, it'll up to triple the overall payload for sending an email, but if that eliminates spam, it'll still end up saving.
--
gfreeman