Outgoing messages failing when internal MS Mail users are copied

[j_son]

My company uses POP/SMTP via earthlink for external mail and MS Mail for
internal mail. Earthlink has had some issues and in order to correct them, I
ended up having to point users out a different gateway (old was watchguard,
new is Cisco ASA). Since the move, a few users get 'No trasport provider...'
error message when they send or reply to a message and address it to both
internal (MS mail) and external (pop/SMTP) users.
Anyone know if any settings in MS Mail or Outlook that I may need to adjust
due to the change? Or anything else that would resolve?

One other thing that may help-I had to switch everyone to authenticated
SMTP...regular SMTP @ earthlink stopped working, they require auth now.

Thanks in advance.

 

[j_son]

Forgot to add that everyone can send/reply to either internal OR external
mails without error, only errors when trying to send to both internal and
external together.