If you are having trouble sending emails to Hotmail, it may be caused by their aggressive SPAM filtering techniques.  If this is the case, you may experience issues ranging from lost emails, bounce backs, and legitimate emails sent to junk folders.

Hotmail's standard response to such issues is provided below. If the instructions below do not resolve the issue you are experiencing, please contact our Support department and we can assist you further.

Hello,

We have identified that messages from your IP are beingfiltered based on the recommendations of the SmartScreen filter.

SmartScreen is the spam filtering technology developed and operated byMicrosoft. SmartScreen is built around the technology of machine learning.SmartScreen's filters are trained to recognize what is spam and whatisn't spam. In short, we filter incoming emails that look like spam. I amnot able to go into any specific details about what these filtersspecifically entail, as this would render them useless. 

Hotmail/Windows Live Mail limits the number of e-mail messages aparticular IP can send within a time period. Based on an IP's reputation(built based on various data sources) it is allotted an allowed sendinglimit per unit of time. When an IP (sender) exceeds its allowed limit, anyfurther SMTP commands from the IP will receive the SMTP error code 421from Hotmail/Windows Live Mail and the connection terminated.

Please confirm that your emails comply with MSN Hotmail's technicalstandards. This information can be found at: http://postmaster.live.com/Guidelines.aspx

You may also be able to find additional information on commondelivery questions at the Hotmail Postmaster Site found athttp://postmaster.msn.com/.

Hotmail has created the Smart Network Data Services program. This is aservice that helps legitimate email senders work with their customersand partners to reduce spam originating from their IP.http://postmaster.msn.com/snds/. This program allows a sender tomonitor the 'health' of their IPs.

Monitor user complaints. Hotmail also has a sender complaint feedbackloop program Junk Email Reporting Program (JMRP). Enrollment in this,free of charge, program will benefit you as a sender as it will keep youre-mail lists updated and populated with interested MSN HotmailCustomers. Participation in this program will remove those MSN HotmailCustomers who do not want to receive e-mails from your company. If you areinterested in joining this program please visit http://support.msn.com/eform.aspx?productKey=edfsjmrpp&page=support_home_options_form_byemail&ct=eformts

Using the SNDS tool, enrollment in the JMRP or having your IPsregistered with Sender ID will not allow emails from your mail servers tobypass our filters, these are in place to help legitimate companiesdeliver their emails to Hotmail Customers.

SenderScore Certified Mail Program. Many legitimate mailers andmarketers have qualified and joined this "white listing" program to improvemail deliverability and decrease email from being filtered to the JunkE-mail Folder. Sender Score is a third party program, administered byReturn Path. Sender Score (www.senderscorecertified.com) is the only WhiteListing service to which we subscribe.  Below are some links which show the reputation results of your IP:  x.x.x.x

Sender Score:

https://www.senderscore.org/lookup.php?lookup=x.x.x.x

Trusted Source: 

http://www.trustedsource.org/TS?do=feedback&subdo=query&q=x.x.x.x

We would recommend working with both Sender Score and Trusted Source in order to build your reputation.

Hotmail has created a group of programs to help avoid future issues I would strongly suggest enrolling-in or implementing these if you have not already.

Segment your mailing infrastructure by IP. Marketing e-mail,transactional corporate, "forward to a friend" e-mail and signup e-mailsshould be sent from different IP's. This will help to identify what types ofmessages are being flagged by Hotmail customers.

Strengthen the sign up process. Confirm that you are using adouble-opt-in sign up process. This will not help in removing existing Hotmailcustomers from your e-mail lists but it will confirm the authenticityof those who sign-up on for your e-mail campaigns and newsletters.

If you have any feedback loops setup with other ISPs you should lookfor trends to try and determine possible causes - a new data source?New advertisement? Maybe folks signing up don't recognize the mail?

Clearly mark your e-mails so that Hotmail customers are able toquickly and easily identify that they requested e-mails from your service.

Do some analysis on the data regarding complaints - look at Hotmailcustomers who have never clicked, opened, responded or bounced in anyway. These poor performers could contain many bad addresses.

Clean up your lists. Remove those who do not want to receive theemails. Make the unsubscribe process more visible.

Enroll in the Sender Score program. This is the only Whitelist whichHotmail uses. It is owned and operated by Return Path. You can findinformation about this program at http://www.senderscorecertified.com.

Please visit http://www.senderbase.org or http://openrbl.org toverify that your IP is not being targeted by any 3rd party block lists. The troubleshooting steps in this email are recommendations only,Microsoft makes no guarantees that following these steps will guarantee deliverability to MSN, Hotmail, or Live.com customers.

Windows Live Hotmail Domain Support