Over the last few years whenever I email a customer of mine in South Korea it always comes back with a permanent failure. Comcast email will not let me connect to South Korea! I've had to resort to using a gmail account, which doesn't have this problem. I really don't like using gmail and wonder if there is some solution.
You need to call Comcast security for this. Emailing to addresses in other countries from Comcast can be problematic.
Comcast Customer Security Assurance-------------------
Normal business hours (6:00 am to 2:00 am EST, 7 days a week)
1 - 888-565-4329
One thing you might try-------you can enable the Comcast account in the Gmail account as an external account, and use it to send from there. I haven't tried it myself, but it's worth a shot.