While Mike is right I decided to investigate in case it was an LMP problem. First, I noticed the "Return-path" was not being set correctly with PHP mail messages (welcome,confirm,notifications), and I tweaked a setting in config.php for that. Still, I couldn't get the confirmation message through to HotMail. Analyzing the confirmation email I saw the subject contained the word "Fr*ee", the text message consisted of a single period "." and then the confirmation email was in HTML. Thinking the asterisk-containing word Fr*ee might have triggered a filter I tried removing that. It didn't help. I then made the confirmation email text-only and that, too, did not help. The solution was to remove the confirm link entirely. I believe there is a block against either links to your entire domain or links to the listmail folder on your domain. What you should do is visit
http://postmaster.hotmail.com and then follow this advice (In my experience you can skip some of the questions if you don't know):
If your e-mail complies with our service guidelines and you are still experiencing e-mail delivery problems that are not addressed in our FAQ, provide the following information and send it to our e-mail address nocmail@microsoft.com
1. What are the IP addresses of your outbound e-mail servers (as seen by the receiving server)?
2. From what domain are you sending?
3. Is your server a dedicated or shared server?
4. What ISP, operating system, e-mail transport software, and/or e-mailing list management software are you using?
5. How are bounce messages (non-delivery notices) handled by your system?
6. How frequent are your e-mailings? What are the volumes of these e-mailings?
7. What are some of the accounts on Microsoft's systems that you are attempting to send to?
8. Do your SMTP logs show any failed transactions when attempting to send to addresses at MSN, MSN Hotmail or other MSN Services? If so, please include them inline in your response.
9. Can you telnet to port 25 of mx1.hotmail.com from your e-mail server(s)?
10. Can you traceroute to 216.32.183.201 from your e-mail server(s)?
11. Include samples of a few messages you are sending, with complete headers (as seen by your recipients.)
12. In what manner(s) are recipients added to your e-mailing list(s)?
13. Provide the URL of your web site.
14. Provide the URL of your Privacy Policy.
15. Provide the URL at which users opt-in to receive your e-mailings.
16. Provide the URL at which users may choose to permanently unsubscribe from your e-mailing list.
17. Are you currently a Return Path client? Are you Sender Score Certified?
18. Do you publish Sender Policy Framework (SPF)/Sender ID records for your IP?
19. Do you use separate IPs for your transactional vs. promotional/subscription marketing communications?
20. Are you in the Junk E-Mail Reporting Program?
21. Do you use Smart Network Data Services?
The most important thing to include, I suppose, is a full copy of the confirmation message -including headers- as received from your server. You should be able to get this from "View Source" in Microsoft Outlook or MailWasher (if you use that to filter email before reaching Outlook).
HotMail should reply back within a day or two with the exact reason your messages are not being delivered.
Regards