There could be two causes for this. The first is actual mail server trouble, where email cannot be sent. In this case, the error message would be surrounded with a red border line. Contact your server admin.
The second possible cause is that the form submission is under suspicion of spam. In this case, the error message would be surrounded with an orange border line.
See also
- I get an error message with a red border. So, how can I solve this?
- Why do I get an error message with an orange border?
- I see a response message “Thank you for your message. It has been sent.” with green border, but I never receive a mail for that.
- Why do you use the same message for mail failure and spam cases?