Search for all customers / partners / vendor / contact / entity record that has a specific email address domain

To search for all Customers/Contacts that has a specific email address domain, follow the steps:

1. Create a search, List > Search > Saved Searches > New > Customer.
2. On the criteria add the field Formula (Text)

Formula:
substr({email},instr({email},'@') + 1)

Value:
is gmail.com

3. Save and run the search.

Note: This formula would work on all entity types, you may just replace the value with the email address domain that you are looking for.