As you may know, we recently had an issue with Windows-based email accounts (Hotmail, MSN, and Live) which were immediately rejecting emails sent from our mail servers with the following bounce message:

Remote host said: 550 OU-002 Mail rejected by Windows Live Hotmail for policy reasons. Reasons for rejection may be related to content with spam-like characteristics or IP/domain reputation problems. If you are not an email/network admin please contact your E-mail/Internet Service Provider for help. Email/network admins, please visit http://postmaster.live.com for email delivery information and support.
Having checked all know spam blacklists, we discovered that although we were not listed, Microsoft had blocked our main email server from sending emails to these accounts.

Rather frustratingly, we could only sort this issue out with them through countless emails and online form-filling. Eventually they lifted the block and could not really explain in any detail why we were blocked in the first place, except to say that:

Messages from your IP are being blocked based on the recommendations of the Symantec Brightmail  as traffic/e-mail originating from your IP matched characteristics of recent spam attacks from compromised, or ‘zombie’ infected, machines.

This would hint that one of the issues that might have caused it was an exploit on one of our clients computers. This could have been a virus that spawned spam emails to the account types in question.

To that end, we are detailing below suggested links in order to check for any infected machines within your organisation.

http://safety.live.com

http://www.microsoft.com/athome/security/downloads/default.mspx

http://antivirus.about.com/od/whatisavirus/a/zombiepc.htm