The Additional Setting “on_sent_ok” is used to assign a JavaScript code that will be executed when a form submission completes and mail has been sent successfully. It is often used for the purpose of tracking form submissions with web analytics services or redirecting to another page after a form submission.

The “on_sent_ok” and its sibling setting “on_submit” are deprecated and scheduled to be abolished by the end of 2017. It’s not that using those settings is unsafe, but it’s possible that enabling them will increase risk in case there are vulnerabilities in this plugin or in other components of your site. It’s time to replace them with a safer alternative.

Update: on_sent_ok and on_submit have been officially removed from Contact Form 7 5.0.

In the Messages tab in a contact form editor screen, you can edit messages that Contact Form 7 displays in different situations. In the messages, you can only use plain text; do not use HTML tags and entities.

Allowing HTML in a message can be a security risk; Contact Form 7 4.4 and later forcibly strip HTML when displaying the message. Review the Messages tab to make sure that you don’t have any HTML there.

We are currently seeing trouble reports in a specific pattern on the support forum. The most common issue is:

• After a form submission, they see “Failed to send your message” with a red border. Mail is not sent.
• They claim that the trouble started after upgrading to WordPress to 4.4.1.