We can see that there are connections coming from your IP, but there are no data packets being submitted. Our logs confirm that your server is establishing a connection to mail.hotmail.com and submitting messages for delivery. It is after Hotmail agrees to deliver your messages that your server then fails to deliver any data packets.
In order to validate or discount the anti-virus software as the culprit can you check the file level anti-virus client software to confirm that you do not have internet e-mail protect or auto-protect enabled? This may be installed on your mail server or on your firewall server. If there is something that looks like "Internet E-mail Auto Protect" or "Internet E-mail Protection" please disable this and then test sending mail to the MSN Hotmail servers again.
If you are currently running Symantec AntiVirus Corporate Edition 9.x on your server, please review the following article from Symantec Support:
We reviewed your domains and DNS zone file and could not find the presence of your SPF record(s). If you would like to receive the benefits of email authentication and validation, you need to create and add an SPF record for each domain. You can easily create your SPF record via our Sender-ID wizard located at http://www.microsoft.com/senderid/wizard Note this wizard provides flexibility to customize your records. To maximize compatibility and deliverability among receiving networks we strongly encourage you to NOT include a "PTR" record (as also discouraged by the IETF in RFC 4408), and to create a record optimized for BOTH the "Mail From" and the Purported Responsible Address, (PRA).
Although, we have added your domain/domains to the SenderID program. This may take up to 2 business days to be fully replicated in the DNS and our systems. If you have any questions regarding this please let me know.
Once you update your domains with valid SPF records you will not need to notify us when you make this or any revision to your SPF record since we will automatically pull the current record from the DNS daily. Thanks again for your support in improving online trust and confidence and if you have any questions or comments please feel free to contact us directly.