Exchange Online MailUser ExternalEMailAddress Attribute changes (Hybrid)

Copper Contributor

Hi Everyone,


i'm looking for a Error in Exchange Online, where my Teams Users have (partly) no Calendar, when their Mailbox is onPremise.


When i tried Autodiscover v2 for effected Users i got a redirect to a Addressspace and not do my onpremise Address Space. Microsoft Told me that the ExternalEMailAddress Attribute at the Mailserver in ExchangeOnline is responsible for that. 


After investigation i saw that some MailUsers in EXO have a different ExternalEmailaddress Attribut that the PrimarySMTPAddress.


While the Investigation with the Premier Support they asked me to create new logs, I tried to enable an account my test accounts and saw that simply enabling the account in my onPrem AD (no further changes) and syncing to AzureAD changed the Attribute ExternalEmailaddress from the primarysmtpaddress to the remote Routing Address from the EmailUser. 


For me it seems that there is an error somewhere in the Sync-Managers in Azure but Microsoft keeps asking me for logs from my onprem Mailserver and running HCW again and stuff like that.


Set-Mailuser -externalemailaddress is of course not working, because it is a synced identity. 


Has everyone seen a same behavoir?





PS: Some Accounts turned themself ok after a while but not all.. 

3 Replies

@pilba Hi, not really in my comfort zone here but have you looked at the prerequisites?


Requirements for mailboxes hosted on-premise (how Exchange and Microsoft Teams interact)


Yes of course. Support confirmed after 8 weeks that there is an unsuspected error in the logs.. 

@pilba Is it possible this workaround is applicable for your scenario?


'Free/busy lookups fail from Exchange Online to on-premises in a hybrid deployment'