Emails via contact form 7 land up in recipients spam box

I have a run a site as a small directory and have more than 100 different customers who advertise on the site WordPress 3.4.1. Some of my advertisers “never” get their mail until i have figured out that most of the mails sent via the site land as spam in their mail boxes. When I am testing the mail and using my email address instead of theirs the mail gets right into my inbox. I am using outlook and a lot of my clients as well and also Gmail. Is there anything I can do to have those mails not going to the Spam boxes? I use the plugin WP Mail SMTP
Kind Regards Lalakoi