I've tried and failed to migrate off Gmail a few times. Mostly due to the number of SAAS accounts I've created with that email address. I like your suggestion to search for "verify your email" etc. I tried searching my Gmail inbox using search you suggested: “verify your email” or “verify email” or “verification”. I got 3 results despite having hundreds of signups since starting the account back in 2006.

Something that would be super useful is a script that can be run against a local Gmail mailbox file that does a comprehensive search for "account created" type confirmation emails and produces a CSV of domain, date, etc. Something that would enable user to delete old accounts, or migrate them to a new email service.