IssueIn Configuration > Notifications > Email Server, there is no "Enable TLS" option in Tableau Server 2018.2.
When sending alerts or subscriptions, errors like the following may occur:
javax.mail.MessagingException: Could not connect to SMTP host: smtp.gmail.com, port: 465, response: -1
or 530 5.7.0 Must issue a STARTTLS command first
And therefore, you are unable to use TLS encryption for email subscriptions with Office 365 or Gmail.