Are you receiving any error messages? With no information, we cannot give any suggestions.

I can't send emails, but I can receive them.

These are the error messages I get: (2 are attached)

Unable to connect

An error occurred during a connection to localhost.

   The site could be temporarily unavailable or too busy. Try again in a few moments.
   If you are unable to load any pages, check your computer’s network connection.
   If your computer or network is protected by a firewall or proxy, make sure that Thunderbird is permitted to access the web.

I have checked everything, but don't know what localhost is.

I can't send anything from my Comcast accounts, either.

Thunderbird worked perfectly for years, and all of a sudden started messing up.

Thanks for any help

localhost errors imply something wrong at isp, not thunderbird.

Thanks for the info. I called Comcast and they had no idea what a localhost was.

What about the other error messages? How would I fix them?

Thanks

Phone support is often done by first-level personnel who are not skilled in the server software. 'localhost' is a term that is part of server software configuration. And that connection error would precede the SMTP error you are seeing. However, I am not a server guru and cannot advise further.