Use SSL to connect to the outgoing mail server in Mail on Mac
If the provider of your email account supports SSL (Secure Sockets Layer, a security technology) for the outgoing (SMTP) mail server, you can ask your provider to supply the information you need to connect to the outgoing mail server using SSL.
In the Mail app on your Mac, choose Mail > Settings, click Accounts, select an account, then click Server Settings.
If necessary, deselect “Automatically manage connection settings.”
Select Use TLS/SSL.
The port number is automatically changed; edit the number, if necessary, to match the information supplied by your provider.
Click Save, then close Mail settings.
Click Save in the message that appears.
If you receive an “Unable to connect” error message after making the change, deselect Use TLS/SSL and contact your email account provider.