Sent Items behavior control comes to Exchange Online user mailboxes

This topic would be nice, but the mailbox owner is really affected if use Outlook for mobile, owa, ActiveSync or Outlook for mac. At least for our tests, not sure if "Mary" in your scenario is happy or not.

Because for each mail owner sends receive inmediately a warning with this message

"The attached message was sent a user who has Send-As or Send-On-Behalf-Of rights for this mailbox. Usually, it would appear in Sent Items, but your service is currently being upgraded. The message will be saved to your Sent Items folder after the upgrade."

Regards