Ok, this one's wierd..
It appears the real From was "From: FirstName LastName, INITIALS < email@example.com >" (spaces added in <> for forum display)
and the MTA in our system that received the email re-formatted it to be
"From: FirstName@mta1.mydomain.com LastName@mta1.mydomain.com, INITIALS < firstname.lastname@example.org >" (spaces added in <> for forum display)
(I see this when viewing the Headers of the message)
So, in my email client it showed "FirstName@mta1.mydomain.com" as though the email message came from MY MTA (which the rest of the headers show it DID NOT, it was properly sent from a remote server, ran through spam filtering, etc).
So, the question comes... I realize the issue was really caused by an incorrect remote mail server who didn't put the email address in a proper format, but why did Zimbra (postfix) do this? If it was technically an incorrect "From", shouldn't it have just been stopped by postfix rather than re-writing it?
What can I do to prevent these type of reactions by postfix to ensure the email doesn't look like it came from "me" when it didn't (Especially since it became an invalid email address - multiple from's - and there was a valid email in the from field)