Content and Inbox Placement

The content of your message is a primary factor in deliverability. Things that could be impacting this are the links in your email message, the number of links in your message, the overall content, and the HTML content and formatting.
Try tweaking your message a little bit each time until it gets to the inbox. Remember, different ISPs take into account the quality of the “mailboxes” as well as the data going into it.