There is no validation of a user's email address during the registration process.
This is a common security measure to prevent bad actors from registering with an email address that is not theirs.
Perhaps you have implemented (in the background)…
enable_stop_forum_spam
"This settings allows you to decide whether or not [Stop Forum Spam] database should be used to validate IPs and emails during new users registrations."
What happens if a member makes a typo in their address when registering, there is no double entry check.
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Misago
Production use
"Site admins may require users to confirm validity of their e-mail addresses via e-mail sent activation link, or limit user account activation to administrator action. They can use custom Q&A challenge, ReCAPTCHA, Stop Forum Spam or IP's blacklist to combat spam registrations. Pletora of settings are available to control user account behaviour, like username lengths or avatar restrictions."
oh… I too cannot reply to threads!