I get an error message with a red border. So, how can I solve this?

The red border means that Contact Form 7 tried to send mail with wp_mail(), but it failed. To solve the issue, you need to find out the actual reason why it is failing in the first place.

There could be various reasons such as the mail setup wasn’t valid.

See also: Red Border Error Issue on WordPress 4.4.1

The sending would also fail if the mail server was down, inaccessible or experiencing other problems. In this case, it will be more difficult to solve the issue by yourself. I would suggest you contact your host for support. They may be able to check the error logs to find out the reason for the failure.

In some cases, it is possible to solve this issue by using another mail server. Postman SMTP is one of the plugins that will help you do that.

Just another contact form plugin for WordPress. Simple but flexible.