That actually sounds like a cool SaaS business idea on it's own.
Create an API to convert email addresses like example@gmail.com to links with custom search parameters like so: hxxps://mail.google.com/mail/u/0/#search/from%3Aexample.com+subject%3AComplete+Registration+in%3Aanywhere
The problem with this approach is that you assume the user only has one email address signed in. They might have `/u/1/` as their personal email used on your website. This could cause more confusion than help since this isn't common practice. (Plus you are assuming gmail.)
Create an API to convert email addresses like example@gmail.com to links with custom search parameters like so: hxxps://mail.google.com/mail/u/0/#search/from%3Aexample.com+subject%3AComplete+Registration+in%3Aanywhere