We currently (and for the past 5 years) have a UC SSL Certificate on our Exchange Server. We are currently configured as a hybrid but all mailboxes have been moved to the cloud. We just haven't actually shutdown the exchange server. All MX and DNS records point to EO as well. The question I have, the SSL is coming up for renewal. Is there any reason that I need to renew this?
I know there is still the Hybrid connector but again, I believe it is just waiting to be removed at this point.
If you had moved all mailboxes/DNS records and all email traffic inbound/outbound to office 365 from on premises , Than you did not require any certificate at all. AAD connect sync did not use any certificate for syncing attributes from your local AD to Azure AD. Certificate only require for email flow and mailbox migration.
Best Response confirmed by
Jeff Harlow (Frequent Contributor)