Hotmail Deliverability Remedies

Today we will be focusing on the theory behind Hotmail deliverability system.

This is the second post after CritSend, we believe in open sharing of deliverability information, and that deliverability is more of a science than a mysterious dark art. Even though our system fully automates these steps and responses and these are integrated directly into our stack, the bulk of them are repeatable for anyone building their own system.

Hotmail Trouble Diagnosis

Hotmail diagnosis system is the best around: it’s called SNDS. You need a Live account and authorize your IP. The first step to diagnose and solve your problem is to susbscribe there. While you’re at it, also create a FBL with Hotmail.

Once you are setup, your screen should look like the following:


All columns are interesting. I want to focus on three data points.

  1. Color Code is the result of the “voting” Hotmail will set up for your emails. Hotmail differenciates different emails sent from the same IP. Red means most of your templates have an issue; yellow some. Green: you’re fine.
  2. Spam Report Rate is the number of people having reported your email as spam. This number must be under 0.1%.
  3. Permanent Bounce Rate should be under 10%
  4. IP status: usually if you see anything there, it is really bad. They will inform you there if Hotmail blocks some of your IP.

Be careful, sometimes it is inconsistent and it has a 24 hours latency. For instance, mails sent from some of our green IP are sent directly into spam. Yet it’s helpful and free. ReturnPath or CritSend’s system would give you more insight but they’re not free.

Deliverability System

The basic of their system follows the same principle we have seen last time:

For each filter, Hotmail differs significantly than Hotmail: here is a quick explanation of which values are checked.

  1. SMTP connection Hotmail checks: IP blacklisting (e.g. SpamHaus, their internal blocklist,…) and SenderID as far as I know. It’s a pass or fail test. Be careful, as if your senderID is badly configured you will see in the 250 SMTP message a “confirmation string” saying that your message has been deleted.
  2. Content Check Hotmail can blacklist a template, a domain and or a mail from. It has also it’s own content filter. For instance, some test message sent from high volume IP will be flagged as spam but not a real newsletter. At this step, Hotmail evaluates DKIM now. To be on the safe side, the mailfrom should match the domain of the DKIM
  3. Engagement Hotmail uses three main metrics: spam report rate (must be under 0.1%), “voting” system (the color of your IP and now some personalized spam filters(they work at the invidual level, so I am not sure if they are relevant here)

Each of these checks are performed per IP and per template.

Limit of the system Issue

Hotmail’s system is improving currently very fast. It is hard to tell how long these issues will remain.

So far here is the main issues we have detected.

  1. It works pretty bad with low volume IP (under 1,000 mails/day). For instance, some of our IP are sending 300 emails on Hotmail and have one spam report. The IP becomes red, while the content is purely transactional (it is an IP dedicated to CritSend’s internal alerts).
  2. One mistake can be very costly: it is fairly easy to get your IP, your domain or your template blacklisted. Once it happens, you have to change them
  3. “A bad start will send you to deliverability hell”. Following the previous items, if you have some minor issues at the beginning of your relationship with Hotmail, you will have a very hard time getting back on track. It is usually easier to change domain/IP/mailfrom.

In a lot of ways, I think Hotmail is the main reason why compnay like SendGrid or CritSend are successful.


To avoid any Hotmail issues:

  1. make sure your email does not end up in the spambox by default
  2. at most 0.1% spam report rate per IP and per template
  3. under 10% permanent failure per IP
  4. daily volume per IP over 10,000 email/day
  5. be extra-careful for the warming up phase of an IP, template, domain

If you experience a deliverability issue, you need to go back to my previous post on Gmail and follow the steps there to fix it. The main difference with Gmail is you might have to change IP once you have successfully addressed it.

Also ReturnPath can certify you. You will bypass Hotmail’s spam filter, have images enabled by default and be allowed higher threshold than the ones I have given you. Usually following these recommandations should be more than enough and ReturnPath would not bring in a lot more. Of course this has to be assessed on a case by case basis

Nicolas Toper

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