Duplicate Detection Excluded Domains Preferences
Administrators can set excluded domains for duplicate detection on the Excluded Domains subtab of the Set Up Duplicate Detection page at Setup > Company > Duplicate Detection.
If you want to exclude certain email domains from Email (domain only) duplicate detection, click the Excluded Domains subtab and enter the domains you want to exclude. Press Enter after each entry. Enter these excluded domains without the top-level domain (.com, .net, etc.). For example, to exclude Gmail email addresses, enter gmail.
Click anywhere on the list, press Enter, and type the domain without the .com or .net ending.