This error horrifies all non-developer contact form 7 users. However you can still take it at ease by following the tips given below.
Check that you have specified a correct email address in “Form” field. As some servers might check the validity of it and its always good practice to keep same “From” and “Reply To” field email addresses.
Most importantly, you get this error because you have used an email addrress which does not belong to same domain name. For e.g. if your domain name is www.kodegod.com then you should use email addresses such as info[At]kodegod.com. So that your SMTP does not filter out your emails.
Just make sure about these two things and try sending the mail again. This has worked for me many times. In case you have anything to say kindly use the comment section below.