As verified with development and product management, we are compatible with SSL/TLS and support mail server using SSL/TLS not STARTLS. Support for STARTTLS would be an enhancement. Please log an idea / enchancement using our Ideation site on Broadcom communities at community.broadcom.com.
If your mailserver/office 365 supports SSL/TLS, you can use it with Applications Manager. But if your mail server / office365 only supports STARTTLS, you can log an enahncement / Idea for the same.