Historically Symantec Encryption Desktop has had the ability to proxy connections for the mail client in order to encrypt emails automatically--these proxied connections have always been POP/IMAP communications. In order for POP/IMAP connections to be proxied, it is necessary for the "Secure" ports be disabled in the email client, such as Outlook 365, because the SED client will intercept the connection and upgrade them to a secure connection automatically. Outlook 365 has made some new requirements that prohibit the disablement of these secure ports, and as that is the case, do not allow these connections to be proxied any longer.
MAPI has never been a supported protocol for the SED client to proxy, only POP/IMAP as per article https://knowledge.broadcom.com/external/article/156008, which contains useful workarounds for this issue.
True MAPI client communication which is normally used for O365 is not yet available for macOS.
Office 365 will not allow any non-SSL communication for POP or IMAP.
Because of these recent changes to the O365 communication channels, we are unable to proxy email on macOS.
Release : Desktop Encryption Any version
Component : Messaging
Symantec Enterprise Division is currently investigating this issue and will report back any new information at this article when it is available. In the meantime, look in the article mentioned above for more information on workarounds.