User cannot receive emails from external addresses

Fix a Hybrid Mailbox with an Incorrect Remote Routing Address

Scenario: a mailbox in a hybrid Exchange environment can send internally but has problems receiving or routing external mail after recipient attributes were changed.

First, verify the remote mailbox instead of changing attributes blindly:

Get-RemoteMailbox -Identity user@contoso.com |
    Format-List PrimarySmtpAddress,RemoteRoutingAddress,EmailAddresses

The RemoteRoutingAddress normally points to the service routing domain, for example user@contoso.mail.onmicrosoft.com. Microsoft notes that you normally should not change this value when Exchange configured it correctly.

If you have confirmed that the value is wrong, correct it in the on-premises Exchange Management Shell:

Set-RemoteMailbox -Identity user@contoso.com `
    -RemoteRoutingAddress user@contoso.mail.onmicrosoft.com

Allow directory synchronisation to complete, then re-check mail flow. Also verify accepted domains, proxy addresses and connectors if the problem remains.

Safety note: make the change only after comparing the affected recipient with a known-good hybrid mailbox in the same organisation.