Lanny,
I'm using POP as well. Sorry I missed that bit of information.
ISP is MacHighway.com.
Ditto the exact same things as Don Hutchinson1 is saying about connection doctor having all green lights, all smtp connections are shown as offline EXCEPT for gmail. I can send via gmail from the Mail app (but not from comcast or machighway). I downloaded Postbox, and it behaves the same way.
For incoming connections all 3 accounts are set to port 995 with SSL.
For outgoing comcast and gmail are using default port with SSL checked; but Machighway just yesterday asked me to select custom port and type in 587 and UNCHECK SSL. I did that, but it didn't change things.
I'm on an early 2012 MacBook Pro and a same vintage iMac. Runing Snow Leopard 10.6.8.
But my iPhone 3GS running 4.3.3 because I use Cydia and am jailbroken and told I can't upgrade (but I am NOT having the same email problem there… email is fine with iOS).
Is there other specific information you need? Sorry this is so confusing, yes, there are 2 of us with what sounds to be the same problem.
Thanks for your help and patience!