NDR issue legacyExchangeDN

Exchange 2013/O365 hybrid setup.
2 domains, AADConnect
A Shared mailbox on O365, synced from an AD user of 2nd domain, is getting lots on NDRs of the IMCEAX type.
Adding the IMCEAX as proxyaddress to the destination mailbox allows emails to be sent.
Problem is, I have a large user base (> 100.000 users) and  just the tracking logs to examone to solve this issue. Tracking logs do not show, as far as i know, the smtp sender but just the undecifrable missinng encapsulated SMTP (i.e. the IMCEAX address which corresponds to the missing legacyExchangeDN value)
So I would like to
- be albe to understand why out of a sudden these addresses generate NDRs (no changes have been done on users as far as i know)
- be able to match the IMCEAX NDR to a corresponding smtp address
- is it correct all mailcontacs on prem (for these users) have a legacyExchangeDN empty? I believe not.
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Pete LongTechnical ConsultantCommented:
I had this the other week :)

Exchange and the LegacyExchangeDN Problem

MichelangeloConsultantAuthor Commented:
Hi Pete, thank you for your pointer!
It does a neat job of summing up the migration process with reference to legacyexchangedn.
Users in my org were provisioned as cloud only mailboxes after a dirsync (live@edu times) with a non-exchange prepared domain.
Recently domain preparation has been performed and hybrid sync activated.
Unluckily I still have to understand the issue root cause and wether a proactive approach can be adopted based on messagetrackinglogs.
MichelangeloConsultantAuthor Commented:
It turns out it is due to a late exchange prepare domain after hybrid aadconnect. not replying to old items dors the trick.

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