Office 365 using Mac - Emails being Sent to Drafts

Hello world - We have 2 users who are having issue with their mail account, wherein emails that they have sent are moving into their Drafts folder rather than into their sent folder. An error message is attached.  I can confirm that the same issue has occurred on their iPhone, from Outlook Desktop, and from OWA.  The error has occurred with simple text mail as well as with mail that includes attachments.

Anyone faced this before and won?