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.
