We noticed incoming emails stopped getting in and we saw a 401 Authentication Error in the CRM Email Router server logs. Nothing has changed in the system account used for this Forwarded Mailbox and the password is still valid.

Deployment: CRM Online + CRM Email Router + Exchange Online
CRM Email Router Configuration: Forward mailbox is used for incoming emails.

After a few tests using the next utilities (very useful!):

https://testconnectivity.microsoft.com
https://soape.codeplex.com

We noticed that system account cannot access its own mailbox. Very weird, like if you don’t have access to your own home suddenly.

Finally, we just had to force an update on this system account to make it work again using the next PowerShell command:

set-msoluser -blockcredential $true -userprincipalname  [userAccountName]
set-msoluser -blockcredential $false -userprincipalname [userAccountName]

It looks like the system account was desynchronised between our internal AD and Microsoft Online AD, causing this issue.