I CANT SEND OR RECEIve emails on my mac pro. I can send and receive on my iphone.

Mail has two groups of settings for mail. One group for receiving and that’s IMAP or POP, and one group for sending and that’s SMTP. Each group of settings has a host name, a port, a connection security setting, and credentials.

Both groups of settings are incorrect. Or there’s a priblem with your network connection from the Mac Pro. There are add-on apps that can cause issues; add-on anti-malware, add-on anti-virus, add-on firewall apps, add-on cleaners, and add-on VPN client apps among those.

Here, you can remove and re-add the mail account and re-add ir, or you can access the settings, and fix the settings for the account directly.

To fix the settings, use Connection Doctor to see what is complaining. In Mail, select Window > Connection Doctor. See what that tool is reporting is working, and is not working.

To get to both the send and receive-related settings (this from Mojave, I don't have a Catalina system handy to check): Mail > Preferences > Accounts > select the problem Comcast account > Server Settings. The outgoing mail server settings somehow do not match the Comcast SMTP server requirements.

In that server configuration, I usually enable connection security, and usually disable the option to automatically manage the connection settings.

Mail > Window > Connection Doctor ...

... click on the red light of any failing server to be take DIRECTLY to the Sending or Receiving server settings (separately, which can be helpful).

If you don't have your ISP recommended settings, there is a Wizard on this Apple web page:

Mail Settings Lookup

You will not get spam from using this wizard, but if you are fearful, just change your login name slightly.