When a user adds a domain to the Approved Senders in Email Impersonation Control, an email coming a subdomain of that entry is still caught by Email Impersonation Control ( EIC ).
Email Impersonation Control
The Anti-Spam approved senders list will also consider any subdomains of top-level domain ( TLD ), whereas EIC does not.
Note: The Anti-Spam approved senders list and EIC approved senders list are unique and independent of each other.
Email Impersonation Control service uses exact match for the Env Sender domain for White Listing and it does not consider / trigger for any subdomains under the top-level domain.
An example is explained below:
In this case it would be better to add the exact match subdomain and in a case where this is not possible because of too many subdomains or because the subdomains are always changing ( typically found in email marketing campaigns and newsletters ) it is recommended to use the Sender IP Addresses entry instead to whitelist the sender from EIC.
The Approved Senders can be edited under https://clients.messagelabs.com > Servcies > Email Impersonation Control Settings.