Email indexing by domain (when enabled) attempts to auto-match Clients based on the email domains in use by Contacts on those Clients. It only executes if an exact match with a Contact in the system cannot be made.
All customer configured blacklists will apply AND there is a long list of excluded ‘public’ domains (such as gmail.com, hotmail.com, and many others).
Example - Client "Blake Enterprises" has two Contacts, firstname.lastname@example.org, email@example.com. If an email is received from firstname.lastname@example.org it will be indexed because it matches one of the domains that was detected for that Client (blake.com, benterpriseses.com).
Domain based indexing is an opt-in feature.
This is a toggle on/off feature that email@example.com can turn on/off at any point for that will apply to all Clients. It will look at domains present on existing Contacts. If there is no Contact with the domain you want indexed on a Client, it will not index emails at the domain level for that domain.