@Muhammad Hassan

You are mentioning that it happens only for few leads

So, Try to Find the reason behind that by checking the Lead details - Like City , State or email domain

You may find the Reason for that 

or else share your Screenshot - so that , it will be easy for us to answer your question

I don't Have enough karma to comment on your answers,
I'm using Odoo 15.

My problem is that a salesperson converted a lead into an opportunity, and the email address of the client is automatically converted into the admin email. it is happening only in a few leads.

I don't know how to reproduce it.

Can u please provide  ODOO Version details -Is it 14? and is it SH

Can you provide a little bit more details? Is this something that can be reproduced? What are the steps to reproduce it? What version / edition of Odoo are you working in?