Can't connect to outgoing mail server (smtp) with Outlook 2007 for smtp.mts.net

Hi -

I have my Outlook client (2007) configured to receive and send email for several email accounts. All seem to work fine except for one (my ISP) MTS Allstream. I can receive fine (mail.mts.net) but can't connect to the outgoing server (smtp.mts.net) when connected to internet via TetherBerry. Might there be some secret portal setting that I am not aware of that will allow connection to the smtp.mts.net server when connected to the internet via TetherBerry?

Any help is welcomed!! Thanks - Operaman