This problem could be caused by your email having several characteristics of spam. Your email might contain images, animations or words that are blacklisted by spam filters. Your email should not be a HTML email and should not contain too many marketing terms.
However, this problem might be caused by your email address too. Some spam filters might identify your email address as a spam email address. Contact your ISP if you suspect that your email address is being blacklisted.
I hope that you find this information useful. Refer to the source article if you need more information.
Source:
http://ask-leo.com/why_is_my_mail_to...g_through.html