WP User Manager sends an email to users after registration. However, this is also send for users that have been created from the backend and other registration processes and often the email is not needed.
To stop it being sent, simply use the snippet in the link below by saving this file to your
/wp-content/mu-plugins/ directory (you might need to create the mu-plugins directory):
https://gist.github.com/wp-user-manager/e1d2452fda1e17af1c055b7e2e7744b6
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