Help with Thunderbird

Cox folks suggested I post here as they ran out of ideas.

I have 2 accounts set up in TB - a Gmail address and a Cox address. Naturally I can send & receive emails via the Cox email address, but for Gmail address I can only receive.  When I attempt to send an email I get this msg:  

"ending of the message failed.
An error occurred while sending mail. The mail server responded: Request failed; Mailbox unavailable. Please verify that your email address is correct in your account settings and try again."

The last communication was to delete the password and then re-enter it.  So 2 questions. 1) is the P/W my gmail password or my cox/yahoo P/W. 2) where in TB do I reenter the P/W. 

of course I have no idea if thats even going to work.  AND if any one has similar TB & Gmail issues, can you share what settings you changed?