If the user in question has a public email address, you can use the appropriate REST API to fetch that information. However, if the user has chosen not to share that information, then it won't be available. Similarly, I don't believe GitHub offers a way to look up accounts by email address.
This is in part because, as we all know, there are bad actors who send lots of spam, and because users may or may not want to link their accounts publicly with an email address at a certain organization (which, for example, could lead to increased phishing attacks against users at a certain company).