Can send one email, cannot send other

Use Thunderbird as my email facility;  Win7.

Changed all passwords all emails recently due to the Recent Unpleasantness with that Credit Bureau;

Passwords changed both email addys with cox and others.

Sent test emails, no prob.

Few days later, tried and failed to send email from one account.  Prompted for new password.  Entered new PW again, test sent, no prob.

Tried to send yesterday and received same prompt;  Entered PW.

Test sent, no prob.

Tried sending today, prompted for new PW.

The other COX email address has no problems sending.

Checked port settings & etc. both, both same.

I see a problem... how does one fix it?  Don't want to orphan email without knowing PW is secure.

Cheers!

CyberDave