Hello Brandon,
MIST should find email addresses from any of the email fields in TntMPD. It doesn't matter if it's listed as primary or not. I use this feature regularly myself... I have a few donors have can't seem to make up their minds which address they like best ;)
When MIST runs the import, does it list the messages with yellow question marks (indicating that MIST sees the emails but can't find the associated email address) or does it not list the messages at all (indicating that MIST doesn't actually "see" the messages)?
If it's the latter, are you using Gmail with MIST? (I notice that you used a Gmail address to send to this list.) Have you read the
Gmail Is Not IMAP page on the MIST site? It's possible that you're only getting the first email in the thread but MIST isn't seeing the other messages at all. Reading that page will give you a technique for solving that problem.
Let me know if that helps!
Tom