We just set a large community-based site up to handle politically-independant campaigns for public users in the upcoming London Mayoral Elections.  To avoid spam and automated signups, we require an activation email to be sent to each user which contains a link to verify the validity of the email address

All works fine apart from one of the largest email providers won't accept our mails : HOTMAIL!!

Lot of people can be found on a google search with the same problem.  One that caught my eye was here : http://www.ecologicalhosting.com/pr/070322_IIS_Aid_Hotmail_Story.pdf

This guy has a full-handle on all his email properties ; MX records, PTR, DNS, reverse-DNS, the lot, and sounds like an established networking guy but it still took him ages to get his mail accepted by hotmail.  He even tested 12 free email providers to find Hotmail as the ONLY one to blanket disallow his server.  He queried it and they replied saying he needed to signup for email scoring by some 3rd party due to the fact they don't supposrt Sender Policy Framework (SPF).  SPF is regarded by most email providers as the email server verification method, yet hotmail choose to use a third party like http://www.senderscorecertified.com/ for a mere $400, oh la la

Further to this, I went and tested my own hotmail account.  I added the exact email address to my "Allowed Senders" list as this is meant to override the spam filter entirely.  The problem is that Hotmail won't even talk to our server for anything more that telling us to get lost with our can of spam :¬(

Hotmail do have a form where you can submit your server to gain acceptance : https://support.msn.com/eform.aspx?productKey=edfsmsbl&page=support_home_options_form_byemail&ct=eformts  

I await the response...