Your definitely should not be using a users submitted email address in the From: field in the Mail tab in the CF7 admin area. If you do, it is now highly likely that your CF7 emails will sometimes get rejected as spam without any warning.
Instead it’s good practice to use a CF7 From: address that matches the domain name where the form is hosted.
You can do something like the following to reply easily to users email address:
To: [email protected]
(use any valid email address on any valid domain)
From: [email protected]
(use a valid email address on the domain where your form is hosted)
Subject: Any text - [your-subject]
(adds extra text to ensure you still have a valid Subject when user does not supply a value)
Additional Headers: Reply-to: [your-name] <[your-email]>
(allows you to use Reply in your email application to reply
to the form users email address)