Microsoft Blocks and Tips to Avoid Them

Overall, poor sending practices with messages going to (aka Microsoft) domains (and any other domain, for that matter) can escalate already problematic junk folder delivery into outright blocking, where your recipient never even has the opportunity to see your message.

No email service provider has total control of or 100% insight into why Microsoft blocks messages to your recipients. The problem is further highlighted by Microsoft doing nonsensical things, for example, like placing their own “Microsoft Certified Professional” newsletters and “Your Xbox Gold Renewal Reminder” emails into junk folders (but somehow all fake “Your PayPal Account is Suspended” emails are in your inbox). It’s their own content being flagged as junk by their own spam filters. When Microsoft sends its own messages into junk folders, all of their bulk sender guidelines are kind of meaningless. If they can’t honor their own rules, it doesn’t bode well for the rest of us. But, there is no reason to compound the problem by not using best sending practices.

At least with junk folder placement, your message was delivered, versus being blocked outright. If your sending is bad, you can expect to be blocked. You can think of blocks at Microsoft as being a type of blacklist.

With the “blacklisting” effect at Microsoft resulting in blocks, basic good sending practices can help alleviate the issue, but is not always 100% initially successful. If you are not sending to opted-in engaged recipients at the Microsoft domains, you’re already behind the eight ball for not having messages accepted for delivery (and then getting into the inbox), on top of being blocked in the first place. What causes blocks, and what steps can you take to prevent it from happening?

Note: Microsoft domains (or, or whatever umbrella brand name Microsoft is currently using) includes Hotmail, Live, MSN, Outlook, and all of the country-based variants ending with ca, fr, uk, and so on.

Lack of IP reputation

If it is a new IP with traffic from you to Microsoft domains, you’re wide open to Microsoft’s best guess as to whether your emails are good/inboxed, junked, or are going to be blocked.

The lack of reputation can be related to the volume. Maybe you have been sending via one or more IPs, but now your volume has significantly increased. There was no gradual build up of volume per day by IP (of course, the acceptable amount of an increase is not known), so Microsoft may start blocking after some messages were delivered. You were considered to be a good sender at your current volume, but the new higher volume effectively makes you look suspicious.

This is typically quickly resolved, and is most commonly due to new content (a variation in spam scoring) or a new FROM address domain is being used (your sending fingerprint has changed). For new IPs, Microsoft typically reviews this content and begins unblocking messages for delivery within 24 hours. But, there are instances where messages will be blocked campaign after campaign, so patience is needed.

Poor IP reputation

A typical diagnostic code for this is:

550 5.7.1 Unfortunately, messages from [fill in the IP address] weren’t sent. Please contact your Internet service provider since part of their network is on our block list (S3140 or S3150). You can also refer your provider to

These blocks can be due to several reasons, and some common reasons are shown below.

Too many complaints from recipients
This is not information that Microsoft makes readily available, but one commonly used rule of thumb is 1 complaint per 10,000 messages sent on an IP is considered bad. Complaints can be minimized by sending to opted-in recipients. It still doesn’t preclude an opted-in recipient from unsubscribing by marking your message as junk, so make it easy (clearly visible, not buried in microprint at the bottom of your message) to find/click an unsubscribe link. Make it easy for someone to click an unsubscribe link versus encouraging them to mark the message as junk because the unsub link is buried or practically invisible.

Repeated sending to unengaged recipients/addresses
In addition to fake addresses, abandoned addresses can “grow up” to be spam traps. From

Trap accounts are accounts maintained by that don’t solicit any mail. Thus any messages sent to trap accounts are very likely to be spam. Well-behaved senders will hit very few such accounts because they’re generally sending to people who give them their address and because they collect and process their NDRs (non-delivery records). Spammers have a much harder time avoiding them because, in general, they can’t and don’t do either of those good practices.

A stale/unused/abandoned address can be turned into a trap account in around 6-12 months of non usage. Repeated sending indicates no list maintenance is being performed, where recipients who do not engage with you are not pruned from your list(s). This is not the (worst) type of spam trap where an address is fake to begin with, although you’ll never know without absolute certainty which type of trap it is. Regardless, why continue to send to people who never open your emails?

Sending unsolicited messages (aka, unsolicited commercial email, or UCE)
An unsolicited recipient is any recipient that has not specifically opted in to receive your messaging. In other words, your message is basically spam, even though your intent and subject line relevance may be good.

Spam-like sending is commonly seen when using the following acquisition sources:

  • Purchased lists
  • Harvested lists from sources such as LinkedIn, Facebook, Google groups, and corporate directories
  • Direct marketing databases who also sell email lists (“Buy an email list and start sending today!”)

Sending unsolicited messages violates JangoMail’s anti-spam policy, and if doing so, your account may be subject to termination. Spam-like sending  will certainly contribute to your being blocked.

Microsoft tools we use for reputation and abuse reporting

In addition to other 3rd party services we use for monitoring IP reputation and blacklisting, Microsoft-specific tools or applications we use include:

Smart Network Data Service (SNDS)
Junk Mail Reporting Program (JMRP)

When receiving a block or complaint, you may see a suggestion from Microsoft to join these two programs. This is not necessary from a customer perspective as JangoMail owns the IPs / servers you send from, and all of the sending IPs are enrolled.

SNDS provides a general overview of an IP’s reputation. The data, across 24 hour time periods/dates, includes information such as the number of trap hits, an example of a complaint (if one exists for a given day), and the overall inbox/unknown/junk percentage indicators (green, yellow, red).

JMRP, Microsoft’s abuse program (complaint feedback loop), reports when a message is marked as junk by a recipient. That “this is junk” action by the recipient makes it way back into our system as a complaint and the address is automatically unsubscribed. As mentioned, all of our sending IPs are registered with Microsoft and no action is needed by the customer for this to work.

In Summary

To avoid blocking (and the junk folder), always follow best sending practices. Ensure your sending mechanics (SPF record, DKIM, and more prominently now, DMARC) are squared away. Check your spam score and keep it low. Send to opted-in recipients. Prune off unengaged recipients. Avoid sudden drastic increases in volume. Consider moving to one or more dedicated IPs if you are in a shared sending pool. If you have any questions, contact Support.