This is likely happening because the email's "from" address domain is your own, but the IP address is Soapbox's. So, there is a mismatch there, and that causes some email clients to flag your emails as possible spam.
To remedy what you are experiencing we recommend you set up SPF records with your DNS host - which is usually your domain name registrar.
What this does is register with your domain name that emails coming from the Soapbox servers IP address are expected and legitimate.
Your domain name provider should be able to help you set those up.
They will need a list of our server IP addresses. Please open a ticket with our support department and we will be happy to provide that list.