Email is not working since upgrade to El Capitan or Yosemite

• If your email client has stopped functioning and brings up error messages instead of sending messages, there may be something wrong with your email settings after the upgrade to Yosemite or El capitan 

• Visit Mail, go to the Preferences tab, and select Accounts. Under the account that seems to be giving you problems, select Account Information, and review the servers used for outgoing messages. Use the proper SMTP Server information according to your latest Internet provider information.

• Much like Safari, Mail may be using extensions that causes the issue. Go to the Mail Menu, go to Preferences, and select Extensions. Uncheck all your extensions and relaunch to see if that fixes the problem. If it does, check through your extensions to find out which ones are causing trouble.