If you need to change which fields are shown or searched on the email reference fields within the email client such as the To field. You can edit a system property "glide.ui.email_client.email_address.disambiguator".
- Navigate to sys_properties.list
- Search for "glide.ui.email_client.email_address.disambiguator"
- Set the fields from sys_user as needed separated by a semi colan.
- Save the record.
There is also a property "glide.ui.email_client.email_address.disambiguator_search" Which can be set to false if you only want to show the additional fields but not search them.