I found someone sends mail by my domain email address.
Most likely not, someone is simply using their SMTP server to send emails under your address. You might want to contact your mail host to verify that and also check out your mail security related DNS entries, to clearly define which hosts are allowed to send email.
Considering Cloudflare does not handle email (except for forwarding), this is unfortunately off-topic for the forum here and best discussed with your provider or on a mail related forum.
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