This happens only occasionally, the type of extern domains, the number of recepients (unless it's multiple recepients), the time remaining in the "drafts" folder, do not seem to be involved in this behaviour. Even with the exact same recepients, sometimes it works, sometimes it doesn't.
When the error occurs, it's either all recepients being flaged as faulty email addresses or none.
Some emails have additionally set a recepient in CC. In a few cases only the CC recepients have been flaged as faulty. This happend with addresses from our own domain.
We've allready checked, that the Outlook client is running in online mode (caching is disabled), and have reset the AutoFill address cache.
Our setup is:
Exchange 2016 cu8 15.1.1415.2 (on premise)
running on Windows Server 2016 1607
The outlook-client being involved is outlook 2016
The 3rd party application support is not familiar with this error. It seems to be a problem with our Exchange server. I'm confused why our local domain is being added to the extern email addresses. Is this related to our local exchange address-policy?