Problem with email

Long ago, I had a comcast.net email account (inactive for more than a year.)

Out of the blue, yesterday, the mail program started trying to "send" my messages through comcast - every attempt to send a message is failing. (since this account no longer exists.)

So, now I cannot send any mail.

How can I eliminate any information about comcast from the mail program (so it will use the correct account?)

I'd appreciate any help you can provide, as this problem is very frustrating.