Every e-mail that I have sent today to friends with a Hotmail account has been returned saying "550 REPLY: 550 SC-004 Mail rejected by Windows Live Hotmail for policy reasons. A block has been placed against your IP address because we have received complaints concerning mail coming from that IP address. If you are not an email/network admin please contact your ..." or something similar. What do I do?
Sadly, the error message is pretty self explanatory: Hotmail thinks you are a spammer.
We'll look at why this might be, and what steps you might take to avoid the problem.
Note that I didn't say "fix" the problem.
Most email services, including Hotmail, are very secretive about the tools and rules that they use to determine what is and is not spam. While we might not like that secrecy, it's on purpose and very important. The reason is simple: if they went public with the rules, spammers could then use that knowledge to work around those rules.
The result? More spam.
So all we can do is make some semi-educated guesses about what's going on.
"... we have received complaints ..." tells me that people have been hitting the Junk button in response to email that originated at your IP address:
"... mail coming from that IP address ..." tells me that Windows Live Hotmail has gotten several complaints, and all of those complaints are about email that originated at your IP address. That can happen for a number of reasons:
You actually are sending spam.
You may not think what you are sending is spam, but the people you're sending it to do. Do you forward a lot of urban legends and other "forward to all your friends" kinds of emails? That's easily considered spam, or at least "junk" by many people.
Your computer might be infected with a virus that's turning it into a spam-sending bot on a zombie network. It could be sending thousands of spam emails without you even knowing about it.
Your computer gets a dynamic IP address via DHCP, and a previous owner of that IP was a spammer. By randomly getting that IP address you "inherited" that spammer's reputation.
Some of these you can control, some you cannot. Taking that same list, we can turn each into a remedy, of sorts, to avoid being flagged as a spammer:
Don't send spam.
Don't send email that people could reasonably flag as junk. Quit forwarding urban legends and the like.
Take all the steps necessary to keep your computer from being infected with malware.
Consider discussing IP assignment alternatives with your ISP.
Those are typically preventative measures, though, and not always something that will help you once your IP address has been blocked or other problems arisen.
One additional source of assistance and information is the "postmaster" domain on many popular email services. For example postmaster.hotmail.com is the "Windows Live Hotmail Postmaster Services". There you'll find topics such as "My e-mail is being blocked by Windows Live Hotmail or is being sent directly to the junk e-mail folder". While much of the information there is technical in nature, it may be of additional help.
Not all email services have "postmaster