The other day I received a Mail Delivery Failed message from clients that were using Hotmail and Outlook.com email accounts.

Those that were using Google, Yahoo or others received the message just fine.

The error details included:

A message that you sent could not be delivered to one or more of its recipients.
This is a permanent error. The following address(es) failed:

SMTP error from remote mail server after MAIL FROM:xxxxxxx@xxxxxxxxx.com SIZE=2169:
host mx2.hotmail.com [65.55.33.135]: 550 SC-001 (COL0-MC6-F24) Unfortunately, messages from XXX.XXX.XXX.XXX weren’t sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com /mail /troubleshooting.aspx#errors.

After researching this a bit I found out that Microsoft has implemented a rather autonomous spam filtering system that as you see in the message, my server/IP address was not blocked due to spam activities , it was simply part of a an IP network.

So this can happen to anyone and apparently is pretty common after doing a quick search on the web. Of course I needed to remedy this issue immediately as this affects any hosting clients on that shared IP that wanted to send email to Microsoft mailing systems.

After searching some more how to remove my server from the naughty list I found the correct submission tool.

Update 9/05/2019: Microsoft discontinued the previous link, here is the updated version.

https://support.microsoft.com/en-us/getsupport?oaspworkflow=start_1.0.0.0&wfname=capsub&productkey=edfsmsbl3&locale=en-us&ccsid=635611717755428181&forceorigin=esmc

It requires a bit of information to be entered, but is probably necessary to make sure its not a spammer trying to unblock his/her address.

After providing all the necessary information I received a thank you for submitting the information and then shortly after received a message the server/IP has been unblocked.

Here is the body of the message:

We have completed reviewing the IP(s) you submitted. The following table contains the results of our investigation.

Conditionally mitigated
XXX.XXX.XXX.XXX

Our investigation has determined that the above IP(s) qualify for conditional mitigation. These IP(s) have been unblocked, but may be subject to low daily email limits until they have established a good reputation.

Please note that mitigating this issue does not guarantee that your email will be delivered to a user’s inbox.

Ongoing complaints from users will result in removal of the mitigation.

Mitigation may take 24 – 48 hours to replicate completely throughout our system.

If you feel your issue is not yet resolved, please reply to this email and one of our support team members will contact you for further investigation.

Within a couple hours messages sent from that system to test hotmail accounts were sent successfully.  Hopefully this experience and information will assist others looking for a solution to the problem spammers have created for even us good guys.