It seems there is an issue with the MailSession object and a specific version of Office365 Outlook (Semi Annual Channel Version 1908 (Build 11929.20776) 32 bit).
The Microsoft Outlook dialog (A program is trying to send an email message on your behalf...) does not open, and MailSession.mailSend returns mailReturnFailure!.
When I changed the windows email account from type Exchange, to use regular imap/pop3 it works.
Anyone else who experienced this issue, and know how to resolve it?