Outgoing mail does not work for me.com or iCloud.com

I cannot get any outgoing mail on account for me.com or iCloud.com. Google allows outgoing mail but not Apple. My iMac is up to date, running Mac OS Catalina, 10.15.2. When I go to "advanced" under Accounts for iCloud.com address it says TLS Certificate None, and then check box for "Allow insecure authentication, which I don't want. How do I solve this? It keeps reverting to smtp.gmail.com. Help please!