List all customer by username

[hiagodotme]

I did an integration of my system with WooCommerce. The only way to identify a user without storing his ID is by email. I did an integration where I create my user in woocommerce as "partner.$MY_SYSTEM_ID", however the only way currently to locate this user is through his email in my system. If my user changes the email I can no longer retrieve his history and I end up creating a new user. I could easily resolve this if I could search for the user by username.