The outlook add-in will suggest contacts, leads, or users based on the email recipient list and the standard email field in Salesforce. It will not be looking for Accounts to match based on a custom field as you're asking.
This knowledge article goes over what it can and cannot do in terms of automatic searching. This, below, is directly from the article and answers your question:
Can this search logic be customized?
Search parameters are not configurable at this time.
Is the domain in the email address used to find an account?
The email domain is not used at this time.
Are other email address fields used in the search?
The standard email address on Contact/Lead/User objects is used in the matching logic. Custom email address fields on those objects will also be considered.
You can search for accounts in that search bar (or other objects) to relate the email to those records, but it will require you to search yourself.