I forgot my mail password and can only recieve emails, can someone help me with this issue please

To reset your password, visit xfinity.com/password. Soon afterwards, Mail should prompt you for the new one.

If it still isn't working properly, the simplest solution is to remove and reinstate the account. Go to System Preferences > Internet Accounts, select your Comcast account, and click the Delete (minus) button underneath to remove it. Then click the Add (+) button and choose Add Other Account > Mail Account to put it back afresh. Refer to How to Set Up Your Comcast Email Address with an Email Program for any settings you're unsure of.

Open Keychain Access and search your login keychain for your email account entry.

Double-click on the entry and click the Show Password checkbox. It will ask for you user account password (the one you log into the computer with). Enter that and it should show you the email account password.

That's a good tip, but is becoming less useful over time as email providers adopt OAuth. If you're using Gmail, Outlook or Yahoo for example, you'll find only a token in your keychain and not your password.

First, who's your email provider? In other words, what comes after the @ symbol in your email address?