(CRM2015) Plug-in - Update field marketing agreement on all Contacts with same email addres

Hello,
I need to make plug-in what work like that:
When marketing agreement(bool) change(true<->false) on contact like "John Smith" with emailaddress1 = "john1@example.com" then plug-in change marketing agreement on all contacts with emailaddress1 = "john1@example.com"

How can I do this?

Thanks