In Exchange 2010 management console, I added a new contact named "Mimecast Journaling", alias "Journaling", email address "journaling@journal.[MYDOMAIN].org"
I started a sync with start-onlinecoexistencesync. I checked Forefront Identity Manager and ran a full import full sync in there as well. No errors showed up in Forefront or in the application log.
Next, I checked using IDFix to make sure that the account didn't have any attributes that caused a sync issue. No problems with the account were shown in the IDFix output--it did show me some other accounts that didn't sync properly, which were service accounts that I have never intended to sync (they use a domain.local UPN instead of a domain.org one).
I've waited a few hours now, and the account still doesn't show up in Office 365. I checked both under Users and under Contacts, in the main Office 365 admin page as well as in the separate Exchange configuration page.
What's my next troubleshooting step? I'd like to use this contact for journaling both on-premises and in Office 365, as I have a hybrid environment right now.