Yes I only have one set up.
Interestingly, If I change the username settings in the Login Details group within the mailbox settings to just <mailbox> rather than <mailbox>@<domain>, the test fails.
However, If I set up my own Email as the inbound Mailbox I can set the username as just <mailbox> and the test succeeds. I have tried also using that as the inbound mailbox and no luck either!