Occasional Error "Remote Server returned 550 5.1.15" when sending to multiple recepients

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In our organization we have a department offering all sorts of courses for local citizens.

Courses, their teachers and the participants are managed through a 3rd party application. The application allows to send informational emails to the participants.

After selecting all participants, writing the email content, an email draft is created within the user's outlook (part of our organization/domain).

Occasionally our Exchange server replies with a invalid recepient email address error:

 

IMCEAINVALID-user+40randomdomain+2Ecom@localdomain.com
Remote Server returned '550 5.1.15 RESOLVER.ADR.InvalidInSmtp; encapsulated INVALID address inside an SMTP address (IMCEAINVALID-)'

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?

 

Has anyone a clue how to resolve this problem?

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