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 > Preferences, click Accounts, then select an account.
Click Server Settings, then click Manage Outgoing Servers.
Click a server in the list, click Advanced, then select Use 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 preferences.
Click Save in the message that appears.
If you receive an “Unable to connect” error message after making the change, unselect Use SSL and contact your email account provider.