I noticed that my Contact Form 7 started to fail tests without a slightest provocation.
This article describes simple steps how to make it work reliable again.
Reliably receive messages generated by Contact Form 7 from any address.
Many SMTP providers further restricted message relay policies. They now only allow messages FROM validate addresses.
Modify your WP email setup
-1- Open “Advanced Email Options”
-2- Set “From Email” field the same as email account you authenticating your SMTP login
Test your WP email setup
Send several messages using “Send Test” on the bottom of “Advanced Email Options” page, and make sure you are getting OK or successful from SMTP server, and that messages are actually received
Modify your Contact Form
-1- Make “From:” field the same as “From Email” field on “Advanced Email Options” page.
-2- Add this line to “Additional headers:” field
Reply-to: [your-name] <[your-email]>
Test your Contact Form 7 page
Make sure you can send messages from your Contact Form 7 from correct, but totally unverified addresses.