Exchange x500 address x500:/o=ExchangeLabs
You have Exchange Hybrid in place. Now you wonder about an address with you see on-premises with all users.
This address has the following format:
x500:/o=ExchangeLabs/ou=Exchange Administrative Group (FYDIBOxxxxxx)/cn=Recipients/cn=234xxxx
This address is now found in the Exchange proxyAddresses in On-Premises as well as in Entra ID. Additionally you will see them in msExchShadowProxyAddresses
The ExchangeLabs is Microsoft internal Exchange Online Organization and therefore needed.
This x500 address is created in Exchange Online and sync'ed via Azure AD Connect (Entra ID Connect) back on-premises. If you delete this address on-premises, the connection between will sync this Cloud leading attribute back to on-premisses.
If you have multiple Domains and Active Directories connected to your Tenant, this x500 address from your tenant will be added to ALL users. Also to users which are from an local AD without Exchange / Exchange Hybrid.
As long AAD Connect/ Entra ID Connect will have the option for Exchange enabled, this x500 will be sync'ed fromm cloud -> on-premises.
In Entra ID: