Contact Form 7 • 15 Nov 2017. Make sure you are using the correct shortcode. “invalid mailbox syntax is used” appears. How to Install Contact Form 7. Contact Form 7 generates a very useful and standard compliant code for the forms. I am using the Drop down menu, with pipe, to enable users to select their recipient. When using Contact Form 7, you will now have syntax highlighting in all form and email edit textarea’s. This tutorial will show you step by step how to resolve some of the Common Contact Form 7 validation / syntax errors. The code [contact-form-7 404 "Not Found"] is a sign that the contact form specified by the shortcode (described in the previous answer) couldn’t be found. This series of articles explains common mistakes and how to resolve those errors. You need to follow the Contact Form 7 syntax pattern so that your emails are sent correctly. All the CSS goes into your theme or child theme‘s stylesheet. When using Contact Form 7, you will now have syntax highlighting in all form and email edit textarea’s. You can not put tags like or