I just dealt with this very issue of email traffic to hotmail dealing with a startup i am working on.
Hotmail sucks as a mail service and microsoft is the devil is the short version.
The long version is,
You must be using SenderID / SPF Records in DNS and register with Microsoft
You must sign up for the Junk Mail program of Microsofts
You must open a ticket with Hotmail support
http://postmaster.live.com to have them unblock you at the filter.
They treat everyone like mailing lists / spam at first.
Messages that come from low traffic domains / servers will likely be dropped,
Short emails are more likely to be dropped,
Having an email address in your contact list will not mean it will deliver mail.
Hotmail will accept messages and report the message is queued for delivery but will never deliver it and not give any warnings or errors so it is impossible to troubleshoot on the sending mail server.
You can validate that this is the issue by sending a message from a hotmail mailbox to a phathack mail account and then reply. that reply will be delivered no matter what. A new email thread will not be delivered if it originates from a non hotmail address.
This impacts all hotmail email addresses (.com or .co.uk for example)
