So mail to any of your customers ends up in the "spam / junk" box.
With hotmail, gmail and yahoo it's importand that you have a correct SPF record. With hotmail your DNS (the MX and mail part) needs to be 100% correct!
It does not matter what OS or mailserver you use. If you do not have a SPF record and a good setup MX / Mail, your mail might (will) endup in the spam folders!
With Hotmail, it will not even arrive!
Test your domain @ www.dnsreport.com
(enter it in the DNS report box)
If you want, than post the results here! A warning or fail (in the MX and mail part) are not good (especially for Hotmail)