@ma_wordpressorg
This is probably because you are setting the From address in the form email to your visitor's email address. Don't do that, or install an SMTP plugin that can handle it. See Contact Form emails won’t send with my Visitor’s Email Address
I already checked the server error logs but didn't get a real help from there.
Neither CF7 nor WP Mail SMTP write to the server error logs.
@andyucs
This message indicates that your mail server rejected your email. See I get an error message with a red border. So, how can I solve this? By installing Postman SMTP you can see the reason the mail server rejected the email in the Email Log.