Exchange 2013 - NDR: 4.2.0 Recipient deferred because there is no Mdb

[adamf]

To outline the setup first...

 

We have our on-premise Exchange 2013 server on which only staff have mailboxes, we use Office 365 for student email and students accounts sync using DirSync.

 

Staff email address are, for example, [email protected] and student email addresses on O365 are [email protected] - there is no hybrid connection between our on-premise setup and O365.

 

Now the problem...

 

For some reason, when staff try and send an email to a student email address they are getting an NDR with the reason as 4.2.0 Recipient deferred because there is no Mdb and I am guessing this is because Exchange is seeing it as an internal mailbox and not trying to relay the mail outside and off to O365.

 

Does anybody have any idea why it would be doing this?