Good evenin worltravlr,
Welcome to the Apple Support Communities!
It sounds like you are having issues with your Comcast email account. If you are changing from a POP to an iMAP account with Comcast, this needs to be done on their end since they are the Email Service Provider. From your Mac, using Safari to go to their webmail page, you should be able to send and receive mail from their website. Once this is working properly and you have the correct email settings from Comcast, then you can add the account to use with Mail on your Mac.
If you can’t send email on your Mac
Mail Settings Lookup
Mail settings you might need from your email provider
How to Set Up Your Comcast Email Address with an Email Program - www.xfinity.com
Add or remove email accounts in Mail on Mac - Mac User Guide
Take care.