How the top 4 email providers determine what is and isn’t spam?

A comprehensive breakdown of Gmail, Outlook, Yahoo and AOL Considerations

Every email provider is a kingdom with their own laws, rules, and customs. If you run afoul of those rules, there is an increased chance of having your email wind up in the spam box. It’s good to know exactly what email providers are looking for when they make the split-second decision as to what to do with the bulk email you send. Below are just some of the things that factor into what determines the fate of where your lands.


Gmail relies on its users and community to determine what to do with the email it receives. The actions of your subscribers and how they interact with the email you send (and email in general) determines where yours winds up.
Here are the numerous factors that Gmail uses in what they admit is an intentionally aggressive filtering system:

  • Activity – If your subscribers report your email as spam, or go into the spam box and mark it as not, do they open it, read it, read and delete it, star it, or reply, it all factors in as to what Gmail thinks of your email.
  • Content – How much this is factored in varies, but content does matter. First, there’s making sure your subscribers receive emails they’re interested in and will engage with. Then there’s having content similar to others. If your email is similar to other emails that have been marked as spam, you can get tied up in that action. There’s always the basics of making sure your HTML is solid, the URLs used aren’t blacklisted, the keywords, images, etc. But, over the years, it has been easier to send what was once thought of as “spammy” words or phrases.
  • Domain and IP Reputation – Gmail looks at both the domain that the email is from as well as the IP it’s using to send to make a decision.
  • Engagement – How much of your list is active, good or bad, matters. Email providers look at how often individuals check email, log in, etc., and it all factors in.
  • Features/Rules – The rules subscribers have, such as sending email to folders, automatically deleting them, or marking an item as high priority impact GMail’s decision.
  • Your Sending History – Google knows… If you’ve received a lot of spam complaints in the past, future emails will most likely go straight to the spam box.



Hotmail like Gmail uses user engagement to figure out how to handle email, but they do it a bit differently which I’ll describe below.

  • Activity – Again like Gmail, how many users are opening, clicking, deleting, complaining, or even doing nothing all impact what will happen to your email.
  • Rules – Like Gmail, any rules set up by users will determine what Outlook does with your email.
  • Sender Reputation Data – This mechanism is similar to complaints, but a lot of weight is made towards the decisions made through this system. This spam fighting system asks users if email sent to them is spam and choose either “junk” or “not junk.” One “junk” classification in this system holds a lot of weight compared to the average spam complaint. We have seen the direct correlation between a negative rating from Outlook and a high percentage of these messages marked as junk.
  • SmartScreen – This technology uses an algorithm to learn what is and isn’t spam. It takes that data from known spam and phishing threats but also customers themselves and Outlook’s Feedback Loop. Feedback Loops are the reporting system that lets email senders know what’s been reported as spam.
  • Volume/Complaints/Spam Traps – Like most email providers all three of these factor into what Outlook does with your email. Strange changes in volume, high complaints or too many spam traps — you’ll have issues getting your email delivered.



Yahoo is interesting in that the email provider filters by IP address as well as the domain, so it’s very important to keep up best practices with this service.

  • Domain and IP reputation – Yahoo looks at the reputation of the IP address you send from + the domain + the “from” address and uses all of that to help make a determination.
  • Engagement – How many users are opening, clicking, deleting, complaining, or even doing nothing all impact what will happen to your email.
  • Trusted Users – Like Outlook, Yahoo has a group of individuals who vote as to whether email they receive is spam or not. These are considered with much more weight than the normal Yahoo user.



Much like the three providers mentioned above, AOL uses a combination of methods to determine what is and is not spam.

  • Engagement – How many users are opening, clicking, deleting, complaining — or even doing nothing all impact the fate of your email. · IP Reputation – AOL looks at how the IP you’re sending from is rated through various services.
  • Spam Vs. Not Spam – AOL places significant importance on individuals marking items as spam or marking spam as “not spam.” Like Gmail, the “not spam” indication is very important and weighed greatly.
  • Trusted Users – Like Outlook and Yahoo, AOL has a group of individuals who vote as to whether email they receive is spam or not. These are considered with much more weight than the normal AOL user.
    So, knowing the above, what can you do to help your deliverability and make sure your messages get to the right people?
  • Onboard Individuals – When an individual joins your list, welcome them to it, reminding them how they got on the list, what they can expect as far as types of messages and frequency, and why your email messages are special and can’t miss.
  • Remove Inactives – Regularly clean your list by identifying individuals who haven’t opened or clicked emails in 6 months or more. Try winning them back with a couple of emails, but those that don’t respond, remove them from your email list. Removing inactive also decreases the chance that your email goes to trusted individuals who may no longer be interested and instead increases the chance emails get in front of individuals that are.
  • Don’t Resubscribe Unless Asked – People click the unsubscribe button for a reason. Unless they have explicitly asked to be added again, you shouldn’t start sending email to these individuals again, because that unsubscribe might then become a complaint.
  • Send desired content – Sending content that’s tailored to the individual’s interests helps increase engagement, which increases deliverability.