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What Do Email Spam Filters Look For in 2017? The Up-to-Date Guide on Getting Your Emails into Inboxes

What Do Email Spam Filters Look For in 2017? The Up-to-Date Guide on Getting Your Emails into Inboxes


That was the sound of your email bouncing.

As a publisher, or anyone involved in email marketing, you might feel there is no worse sound. Except there is. It’s the sound of silence.

It’s the silence of your email never even reaching the recipient’s servers - or reaching the mailbox, and being swiftly and silently escorted to the spam folder by the automatic filters. Because there was no BOING! - you never know.

The percentage of sent emails that actually make it to the inbox (your deliverability rate), is the metric by which email campaigns and email newsletters live or die. It’s not just related to that one email, either. One email campaign with outstandingly bad deliverability (think 40-50% of your emails never made it to their destination) and your future campaigns start with a mark against them.

What does your email need to do to make it to your recipient’s inbox? Let’s take a look at your email’s path and the checkpoints it needs to pass through safely. (Hat tip to Return Path for their incredibly comprehensive guide on how email spam filtering works and to Vision Design Studio for their way-helpful-for-the-layman blog post on how email delivery works. The image below is theirs.)

How email works (in a nutshell)

  • The sender uses the mail client on her computer (i.e. Outlook, Gmail, Hotmail) to write an email.

  • When she presses “Send,” the email is sent from the mail client to the email server for that domain ( addresses have a different email server than addresses which have a different email server than addresses which have a different email than addresses.)

  • The email server locates the recipient’s email server and transfers the email over.

  • The recipient’s email client downloads the email from the server and displays it in the inbox.


So there are effectively 3 points of transfer.

  1. Sender’s email client to sender’s email server.

  2. Sender’s email server to recipient’s email server.

  3. Recipient’s email server to recipient’s mail client.

Where can emails get stuck?

The last two points of transfer can (and usually do) have filters.

When the email is transferred from the sender’s email server to the recipient’s email server, filters are likely checking the emails just before or after they enter the “gateway” into the server. If the filter flags the email as spam or malware, it will never make it into the server, or be deleted immediately from the queue waiting to be transferred to the email client.

(Barracuda has a very clear one and a half minute explanatory (albeit promotional) video that’s an excellent way to understand how gateway filters work.)

If the email passes the email server’s filter, it still needs to get past filters that guard the way from the recipient’s email server to the recipient’s mail client.

These are the filters that evaluate where in your mailbox the email should be placed. The inbox? The spam folder? The “Promotions” or “Social” tab, in the case of Gmail? Some of these filters are general, for all users of the mail client. And some of these filters are custom filters that you have created.

Look at all the filter options Gmail gives you:

email filters.png

If your email makes it past the server filters all the way to the mail client, but is there identified as spam and placed in the spam folder - you’ll never know. Almost all email marketing providers will report that email as “delivered,” because they can only track bounces (when the email is temporarily or permanently rejected by the recipient’s server).

How can you increase the chances that your email will REALLY be delivered - and show up in a place of honor in your recipient’s inbox? What do email spam filters look for?


The more recipients complain about receiving your emails (either by marking them as spam, or by complaining directly to their mail client providers), the more that mail client provider is going to be wary about passing your email on to its users. In other words, if a bunch of Gmail-using recipients all mark your email as spam, Gmail is much more likely to mark your emails as spam in the future - for any of its users.

What To Do?

  1. This falls into the “obvious but overlooked” category: only send emails your recipients WANT to receive. Sending unwanted emails will not only destroy your relationship with the recipient, but will harm your chances of reaching recipients in the future.

  2. Make sure it’s easy to get to your unsubscribe option or preference center. Even if recipients originally wanted the content, if they change their mind and it’s too hard to unsubscribe, they may mark your email as spam as a way of not needing to see your emails anymore.


A bounce is when your email is rejected by the recipient’s email server, either because the email address doesn’t exist or is no longer in use (“hard” bounces) or because the mailbox is full or the server is experiencing temporary difficulty (“soft” bounces). A high rate of hard bounces indicates that you’re not keeping your list up-to-date or being careful about how you get your email addresses. Email providers are going to be wary about letting your emails through.

What To Do? 

  1. As above, make it easy to get to a preference center and update your information. If you’re sending emails your recipients really want, they’ll update their email address should they move away from the old one.

  2. Maintain good “list hygiene.” Go through your subscriber lists regularly and prune out any email addresses which have been bouncing emails (some email service providers do that automatically).

  3. Use double opt-in (also required by some email service providers). When a subscriber has to take an action (like clicking the link in your email) to be added to your list, the chances that it’s a valid, active email address are much higher.


If you’re bad enough, your domain/IP address might get put on one of the spam blacklists. Email servers and email clients will often use at least one of these lists when checking incoming mail. If the sender or a link in the email matches one of the domains on the blacklist, your email will be rejected.

Blacklists are created from domains that are known to send spam, and from spam traps. Spam traps are email addresses that are no longer in use (or were never in use, but were left hidden in the code of websites for spam robots to find.

What To Do?

  1. Check the blacklists. If you see your domain on one, contact the coordinator if possible, and request to be removed.


You might come out of the woodwork with a valid, vibrant 50,000 people list… but chances are you won’t. Most natural email lists grow gradually.

What To Do?

  1. Start off slow, and build up the volume you send over time. Which brings us to...

Sender IP Address Permanence

Spammers often jump from one IP address to another in order to avoid the IP address they managed to get blacklisted. Fortunately mail clients are smart, and when they see a large incoming volume of mail from a brand-spanking-new IP address, they’ll only let a small amount of it through to inboxes, so they can see what the reaction is.

What To Do?

  1. Warm up a new IP address. Even if you’re starting out with a list of thousands of subscribers dying to hear your words of wisdom, send first to a sampling of them, then increase slowly. Otherwise your words of wisdom are unlikely to make it to ANYONE. 

  2. Stick with one IP address. Consistent use over months and years will build your sender reputation.


Spammers can send emails that look like they were sent from a trustworthy domain (e.g. or With no authentication, there’s no way to know, and mail clients will be very suspicious. But if your sender email has authentication set up, the recipient’s mail client can confirm the ID of the sender, and know that it’s not some spammer mimicking your email. Most reliable email service providers will give you at least basic authentication of SPF and/or DKIM. DMARC is an even stronger authentication method. With the authentication methods that are connected to your domain (like DKIM and DMARC), your reputation will move with you even if you change email providers/servers.

What To Do?

  1. Make sure your email service provider has at least one of these three authentication methods: SPF, DKIM and DMARC. The more, the merrier.


Ironically, what’s actually in your email matters less than all the above reputation factors. A strong reputation will be able to push through emails that look a bit shady, while innocuous looking emails that come from a shady sender will land in spam.

Content does play a small role in email filters, however, so take care to keep your emails looking good.

What To Do? 

  1. Don’t use only images. Balance your text and images so people can understand your email even if they can’t see the image.

  2. Make sure your HTML isn’t broken.

  3. Don’t stuff your email with links.

Subscriber Engagement

Are your subscribers reading your emails? Are they so excited by what you wrote that they reply, forward it to other people, or click on the links within? Or do your emails just sit unopened in the inbox until they get deleted?

Those are some of the engagement metrics that email clients look at when they decide if your emails are worth putting through to their users in the future.

What To Do?

We’ve said it before, but we’ll say it again: only send emails your subscribers WANT to receive. Fewer, more targeted emails are better than bombarding your entire reader base with every email you could possibly send.

  1. Write compelling subject lines that motivate your reader to open the email.

  2. Take advantage of the first words of the email that often show up in the inbox. Give a tantalizing preview of the can’t-miss info your email contains.

  3. Encourage engagement. Ask your readers to click reply and shoot you an email if they have feedback. Suggest that they share the information with contacts who would find it useful. Be smart about your calls to action. Make your links clear and easy to click. 

What Do Email Spam Filters Look For?

ReturnPath analyzed the rate at which emails from reputable email marketers (not spammers) actually got into the inbox. Since Q2 2015 until Q2 2016, global inbox placement rate averaged 79%, with the USA coming in under the global average at 73%.Kudos to email marketers in Australia, Canada and the UK, with inbox placement rates of 88-90%.

Want to be in the top percentile and move your inbox placement rate closer to 100%? Follow the recommendations above, and your emails will be on their way.

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