After 6 hours of Trial & Error, pulling my hair out, and searching every FMP resource I could get my hands on (I ALMOST called FM Tech Support, but I'm too stubborn and cheap to spend the $45), here is the solution I discovered which fixed the problem to what I consider a Filemaker "BUG" in the SMTP Server 'Send Mail' Script command.
When you Configure (or 'Specify') the SMTP Server in the 'Send Mail' Script Step, DO NOT enter a NAME in the User Information field.
For some reason (unknown to me) when you enter a NAME in the User Information field and send the Email it is converted into UTF-8 (Unicode Transformation Format) and it looks like this: Sender <=?UTF-8?B?RmlsZU1ha2VyIFNlcnZlcg==?=<firstname.lastname@example.org> which results in the following error, "553 Requested action not taken: mailbox name not allowed"
The Sender information should probably look like this: Sender <Joe Hockeypuck><email@example.com> not Unicode
I hope this helps someone out there.