Enable AD Connect sync with existing Office 365 accounts and mailbox

Hi, I'm managing an Office 365 tenant with 60 users with E3 subscription. For the moment all users have a username with @mydomain.onmicrosoft.com domain and a mailbox associated. Now, I want ...

exchange

hybrid

office 365

dan_jjjj

Copper Contributor

Aug 11, 2020

When I try this, I get an AttributeValueMustBeUnique error when it tries to syncronize for the new user since the UserPrincipalName is not unique. I followed the guide on this page...

https://support.microsoft.com/en-us/help/2641663/use-smtp-matching-to-match-on-premises-user-accounts-to-office-365

Do you know how to get around this?

p.s. Sorry for posting on an old thread. It was one of only a few that I could find that directly discusses AD Connect to an existing 365 company.

Thanks