Emailed you some tips, but will post here as well
What emails are marked as spam is mostly determined by the recipient mail administrator and is not something you as the sender can always control. Generally the best you can do is create mail with accurate subject lines and full headers (such as From, Reply-To, Sender, for HTML messages MIME-Version and Content-type). Unfortunately, mail servers are extremely adept at detecting where mail was sent from (mail client program vs web server) and often assigns scores partially based on that alone. You can encourage your recipients to white-list your domain so that emails from your domain are less likely to be sent to their spam folders. You may want to consider switching how your domain sends out emails; emails sent using SMTP are sent directly from your mail accounts on the mail server and are DKIM-signed.