There are various anti-spam measures that can be employed to limit spam. None are likely to block all spam from your inbox and all are likely to come with some risk that legitimate email will be blocked as well. When a legitimate email is blocked as spam, the block is considered to be a "false positive".
This email server like many others uses DNS blacklists, aka blocklists, to attempt to limit spam coming into users' inboxes. Every system transmitting data over the Internet has an Internet Protocol (IP) address assigned to it. This address is commonly assigned by an Internet Service Provider (ISP). For home users it is unlikely to be a fixed address and may be different each time you connect to the Internet or is at least subject to change frequently.
When two email servers communicate with one another, each must know the other's IP address in order to send information and respond to one another. Some organizations maintain lists of IP addresses from which their email servers or others' email servers have received spam. Those lists are constantly updated as the systems sending spam continually change. Many of the organizations maintaing such lists make their lists available to others in a communal effort to combat spammers. System administrators of email servers may choose to use one or more such lists to limit spam coming into their own servers.
Each time an email message arrives, the IP address of the sending system is checked against any blacklists employed by the receiving email server. The receiving email server will contact a Domain Name System (DNS) server of the organization providing a blacklist used by the receiving email server with the IP address of the system attempting to send email to that receiving email server. If the DNS server indicates the sending server's IP address is on a blacklist, any email from that system will be rejected by the receiving email server.
One blacklist is the Spam and Open Relay Blocking System (SORBS). Unfortunately, at least several of Microsoft's Hotmail email servers are currently on a SORBS blacklist. Since email from MSN.com email addresses is also sent by Hotmail.com servers, email from an msn.com email address may also be rejected.Questions
With the default Hotmail account settings, you won't see the reason the mesage bounced listed. To see the reason for the message rejection, you have to take the following steps while logged into your Hotmail account.
You should now see something akin to the following in the message.
Diagnostic-Code: smtp;550 5.7.1 <email@example.com>... Mail from 220.127.116.11 refused - see http://www.dnsbl.us.sorbs.net/
Look for the "Status" and "Diagnostic-Code" values to determine why the message was rejected. In this case, the message was rejected because it is on the SORBS blacklist.
If the IP address of the system attempting to send the email is
listed in the SORBS blocklist, then the mesage is rejected with the message
Mail from xx.xx.xx.xx refused - see http://www.dnsbl.us.sorbs.net/
with xx.xx.xx.xx being the IP address of the sending system. Note that
it is up to the administrator of the receiving email server as to what
text, if any, is included when the message is rejected. Many choose, like
I do, to mention the blacklist that was used by the server to reject
the message, but some may not provide any details as to why the message
was rejected. So, if the status code is 5.7.1, but you don't see a reference
to SORBS, it is still possible that email is being rejected from your Hotmail
account by a server using the SORBS list, even though the rejection message
doesn't actually mention SORBS.