I find it interesting that you didn't have problems with email sending prior to 11.2 since you use tls. (Since I was unable to get it working without the latest change, having tried various resource combinations).

This is our resource that works:

<Resource name="mail/Session" auth="Container"
       type="javax.mail.Session"
       mail.smtp.host="mail.example.com"
       mail.smtp.user="xxxxx"
       mail.smtp.password="xxxxx"
       mail.smtp.port="587"
       mail.smtp.auth="true"
       mail.smtp.starttls.enable="true"/>