In a cloud based D365fO environment emailing using exchange is only allowing using the same tenant as D365fO is hosted at. This means users can only belong to one tenant unless you change the SMTP setup to something that allows just about everything to be emailed. This may result your SMTP to be banned sooner or later.

Enabling support for multi tenant emailing would allow external users to use their own Office 365 accounts helping onboarding and speed up users at acquisitions / adding external outsourced users (like external warehousing) as well as adding external consultants with full functionality.