Sorry for the late reply. This seems to be slightly beyond the scope of our support (it looks like a server configuration issue), but we will look into it anyway as time permits because it would be useful to know. We know for sure that impersonation works in some configurations in Exchange 2010 and later, but we are not sure about Office 365.
Alternatively, you might try using Rebex Ews object (part of Rebex Secure Mail as well) to access Office 365 - it has an API similar to the Imap object and supports impersonation as well.