About:

User sometimes created in exchange on-prem and migrated to O365. Then a settings or attributes changes in the AD account. Suddenly the user cannot receive and sometimes cannot send also to external addresses.

You will need to check remote user attributes in exchange on-prem.

You will need to change the routing address to the .mail.onmicrosoft email address to make it route to the correct one in O365.

Step:

You will need to connect PS to exchange on-prem and run the command below.

Code:

Set-RemoteMailbox -Identity “doconnor” -RemoteRoutingAddress ‘doconnor@RidleyOnline.mail.onmicrosoft.com’