Make sure you have reverse DNS that points at your mail server ( eg mail.your-site.com ) , not just the root domain ( eg not your-site.com ). A lack of reverse

Make sure you have reverse DNS that points at your mail server ( eg mail.your-site.com ) , not just the root domain ( eg not your-site.com ). A lack of reverse DNS or an automatically generated reverse DNS ( eg isp-1-0-0-127.isp-site.net ) won't do.