ProxyAddressExistsException: The proxy address “SMTP:kevin.sims@domain.com” is already being used by the proxy addresses or LegacyExchangeDN of “Joe.Simon_domain.com#EXT#”

The following error may occur when syncing to o365:

ProxyAddressExistsException: The proxy address “SMTP:kevin.sims@domain.com” is already being used by the proxy addresses or LegacyExchangeDN of “Joe.Simon_domain.com#EXT#”. Please choose another proxy address.

This error occurs when the sync attempts to ADD a contact (email address) which is already being used by an existing o365 object. The new object will not be added, therefor an Error is logged. Subsequent Syncs will continue to attempt the Add.

As of v4.1.37, if the destination object is a Guest Account, a warning (rather than error) is logged.

You may recognize Guest accounts if the logged message includes Guest or #EXT# in the identity.

i.e.
“Joe.Simon_domain.com#EXT#”
or
“Guest_b94fee43ddea4fb6a8dd0183a0e64001”