User shows up as mail user onprem and mailbox user in o365

[Jason Benway]

we have hybrid exchange 2016 to o365. AD connect up and running for years.

We are bring in an user that had be an external sales person to be in employee next week. They had their own domain and we are bringing that into o365. 

They were setup as a mail user forwarding our domain.com to his domain.jp, no problem.

I was prepping for next week and notice that onprem he's a mail user but somehow he has a mailbox in o365. The attributes seem to be out of sync for this user. I checked and there are some emails in the o365 mailbox! even recent ones... So I don't want to just blow it away.

I need to get the onprem server to know he's a o365 mailbox user. Any ideas?

[#2]

What kind of object, if any, exists for this user in your AD?

[#3]

He's an user object in AD not a contact.

@Vasil Michev 

[#4]

So I take it he existed as user object in AD, synced as mail user in ExO, right? Then someone assigned a license to it in ExO and provisioned a mailbox? If so, you will have to populate the relevant attributes manually on-premises, including ExchangeGUID, msExchRecipientTypeDetails, msExchRemoteRecipientType, msExchRecipientDisplayType, targetaddress, proxyaddresses, mail, mailnickname, and probaly few others I'm missing.

[#5]

oh boy that sounds fun :)
and I doubt o365 support will help since its a hybrid on prem piece... urg. I'll dump the exchange details of an user that is working correct and this user and compare.

Thanks!!