I would like to clearify that I can pickup mail successfully from the external client - its just sending that I'm having a problem with.
To make sure that I'm on the same page -
The mail server for example is mail.123.com
I have the main domain being 123.com with user accounts
I also have domain 456.com with user accounts and domain 789.com with accounts.
If I want to log into an account on the virtual domain 456.com - I'm using
username@456.com as the ID with the associated password. From the web interface, it works perfectly - no luck from an external client from an external location.
I'll try upgrading to M3 to see if that has any effect..