Sometimes it happens that some emails do not enter correctly in Hotmail, the email arrives but goes into the junk mail.
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It is advisable:

- Check SPF rules.

- Check domain keys.
- Check that the IP of your connection is not on an important blacklist. http://www.mxtoolbox.com
- Check that the sending IP of the server is not on any important blacklist.
- Check that the server IP has the reverse IP configured.
- Notify MSN of the sending of emails by your domain. https://support.msn.com/default.aspx?productKey=senderid&locale=en-us&st=1&wfxredirect=1
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And some more aspects:
​http://mail.live.com/mail/troubleshooting.aspx
One way to ensure that your messages are not marked as coming from an "unknown sender" is to join the Return Path Sender Score Certified Email program (http://g.live.com/9wc9en-us/senderscore), an accreditation and reputation service from other providers that provides Windows Live Hotmail with a list of responsible senders. Alternatively, if a Hotmail user adds your domain or email address to their "contacts" or "safe senders list", they will no longer see this notification. In addition, users who are on the Sender Score Certified list from Return Path or on a user's "safe senders" list usually have links and images in their messages enabled by default.