Same here. I just enabled with Microsoft Exchange Online and all mails fail that have (another) "+" in the tag-component of the mail address. Error Message is 550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient
Very annoying and I would be eager to have a solution.
Does anybody know about a problem report filed with Microsoft? Any other links addressing this issue?
I cannot think of any other solution as having Microsoft fixing the issue or Moodle avoiding this character.