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.

No bugs have been found so far relating to this issue, in WordPress or the Contact Form 7 plugin.

After looking through a lot of reports, I found that some of them had misconfigurations in their mail setup, and they could resolve the issue by correcting the configuration. It is possible that misconfiguration in mail setup is the main cause of this problem in many cases.

A large part of the misconfiguration is seen in the From field in their mail header. The From field is not a label or decoration; it must be a valid email address that is in the correct syntax. Otherwise, mail servers won’t be able to deliver the mail.

Following the standard specification, a From field value should be an email address:

john@example.com

It can also be a name and an email address surrounded by “<” and “>”:

John Smith <john@example.com>

While you can use Contact Form 7’s mail-tags in the field value, I recommend not using mail-tags there unless you completely understand how they work. Using an email address is safe.

Which email address should you use in the From field? You should use an address belonging to the same domain as the site. The best practice is using wordpress@{your-site-domain} and preparing the address on your host.

You might be wondering why you had been able to send mail even with invalid mail setup. Why did this issue suddenly start with WordPress 4.4.1? We’re not certain, but the important thing is that mail header fields must have valid values regardless of WordPress versions. This is a good time to review your mail setup.