Hi @HubSpotMaster,
Great use-case here! The option to do this via a workflow would indeed not be possible as you’re attempting to work with very specific values on multiple records in your account.
A secondary option to working with instead of the Email address is to work with the Website URL property found in a Contact Record. When this is property is filled, it can be used as a secondary unique identifier which will allow for a contact to be associated to a company, should the Website URL match the Company domain name property found in the Company Record.
In this case as an example, if I were to add myself as a contact under the address ‘mwillson@gmail.com’ but also pass the Website URL as ‘hubspot.com’, we would see my record created and the Associated Companies updated to show HubSpot - this is because the Website URL matches the value of the Company domain name.
I hope this helps in your scenario and happy HubSpotting!