Favicon were added so that the user can directly link the sender of an e-mail to a company, without necessarily needing to carefully check the signing domain and from address.
As currently favicons can only be shown for whole domains, this means that e-mail provider can not be added to the list (unless they use a separate domain for their employees and official e-mail addresses).

Adding the possibility to do show favicons also for specific AUIDs and From addresses would change this. The add-on could show the favicon only for official e-mails (e.g. support@e-mail-provider.example or employee_name@e-mail-provider.example with AUID set to employee@e-mail-provider.example).