Exchange hybrid - on-premise mailbox issue

Copper Contributor

Hi Guys,


I have a problem with mailbox that is located in on-premise Exchange 2019. I'm using hybrid mode with 99% mailboxes located in EXO. Let's name this mailbox: "Mailbox1".


Mailbox1 have 4 SMTP addresses. 3 of them are working correctly, only 1 is not working- mails sent on this SMTP are not delivered with error:

: ‎550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient XXXXXXXXX not found by SMTP address lookup‎

While I'm sending mail from this specific SMTP mail is delivered correctly - reply on this message is also not delivered.


I have found some discussions that could be related to this issue. For example:

Accepted domains in EXO 


All my accepted domains in EXO are configured as Authoritative. But I'm not sure that this is the root cause of this issue because it's working with 3 others SMTP for same mailbox and also some other mailboxes from on-premise are working correctly(some- not, same issue as described).


Do you have any experience with issue similar to this one?





5 Replies

There are 2 objects in cloud:
1. MailUser without required SMTP address added
2. MsolUser with all SMTP synced from on-premise

I'm not sure why on-premise mailbox is synced to MsolUser instead of MailUser.
Both ExchangeGuid for on-premise Mailbox and MailUser are same.

Any ideas?
Do you see the SMTP address listed for the MailUser object in EOL? Run in EOL==> (Get-MailUser Mailbox1).EmailAddresses
The address you're using have the correct email type "smtp:"?
SMTP addresses that were added into mailbox(on-premise) are sycned to MsolUser object instead of MailUser object in cloud. Any ideas how to fix that issue without excluding object from sync and deleting both objects in cloud?
Not sure that you have this correct. MSOLUser is the user in AAD (found using get-MsolUser cmdlet). The mail user should be the same user but found in Exchange Online using the get-mailuser cmdlet. If you have TWO users found in AAD (and possibly EXO) with the same details, then you have a problem. Check the AAD Connect health to see if there are any sync errors. You might see an indication of duplicate addresses or user type mismatches


The bounce back is returned when the recipient address isn't available on the server where your MX is pointing to. 

Please confirm if the 4 SMTP addresses on Mailbox 1 are of the same domain name. 

Please confirm if there are other working mailboxes (Mailbox 2, mailbox 3) with address like the affected domain name on Mailbox 1

I look forward to your response in order to advise further.