You have to make sure that all emails are sent 'from' the same address as your domain name. By default, Contact Form 7 doesn't do this, it uses the email of the person filling in the form. Our Anti-Spam Policy means you cannot send email from an address that isn't your domain.
If you install http://wordpress.org/extend/plugins/configure-smtp/ on your site, no need to configure it, just install and activate, it should fix it. The other way is to make the 'from' always be your domain, but 'reply-to' is the person who submitted the form.