If you could paste the exact error message you're getting that would help.
Ambari uses JavaMail to send alerts via SMTP. When creating/editing a notification in the UI, there's a section all the way at the bottom where you can "Add Property" - it should be right below the "TLS" checkbox. Here, you can supply any JavaMail property you need.
For example, you could add the property "mail.smtp.ssl.trust" set to the value of "*" (without quotes)