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Spam filters & deliverability: What mailbox providers expect from email senders

How can you use sending best practices to stay on the good side of mailbox providers’ spam filters – and make it into the inbox?



Spam is everywhere. Literally.

You’ve probably noticed this yourself, just by looking through your inbox, but what actually gets through to you is a minimal fraction of what gets sent every day. Mailbox providers see a colossal amount of spam – or worse, fraudulent or phishing – emails coming their way. It’s a bigger amount than you might be able to imagine, and that’s why mailbox providers need spam filters that do a really good job at identifying these messages before they reach your inbox.

As a recipient, this is great news. But as senders, it’s sometimes frustrating to navigate the complex world of email deliverability and understand what these providers are actually monitoring… and why our emails are landing in spam.

So, how do mailbox providers handle spam filtering, and what does it mean for you?

Why do mailbox providers use spam filters?

Every day, Gmail alone blocks more than one hundred million phishing emails that try to steal information or money from Gmail users. The Talos Reputation Center provides monthly reports about the daily amount of spam seen worldwide, and it doesn’t look pretty:

No matter the actual volume of phishing or spam emails that the mailbox providers are receiving, what we can be certain of is  that it is their responsibility to protect their users against those unwanted (and sometimes dangerous) emails. No email user wants to receive an email that might steal their money or information… nor would they like to receive unwanted communication.

Gmail has said that they block 99.9% of spam, phishing, and malware from reaching Gmail inboxes. So you can figure, from that perspective, that you want your own email messages to be in the small percentage of non-spam that makes it through. 

We can understand from Gmail’s figures that it must be quite a difficult task for any mailbox provider to determine what the “good” emails are out of the vast amount of emails they’re receiving daily.

But it’s in the interest of those mailbox providers to protect their users against any form of spam or abuse. They would lose their customers to another mailbox provider if they weren’t doing their job properly.

Okay, so you now know why inbox providers act the way they do. But what does it mean for you?

What mailbox providers expect from you

You're probably thinking the above doesn’t matter to you. You’re not a spammer, and you’re certainly not a fraudster. Your emails are legitimately sent to your customers or subscribers. And more importantly, they bring value to your audience. They deserve to be accepted by the mailbox providers and your recipients and be placed in the inbox, right? All should be good in the world. 

But the thing is, you need to think about what those mailbox providers have to deal with every day, and how it might affect deliverability in general. Do they see a very large amount of junk mail every day going to their users? Well, yes, definitely. Do they have to be strict against incoming email in order to protect their users? Again, another yes. 

So how can they determine if your emails are more spam or phishing in the sea of email they’re seeing every day on their end and what can we do to help them put your emails in the inbox where they should be, and avoid the spam folder? 

Aim for a positive audience response

There are many factors that come into play when we’re talking about email deliverability. But if there’s one that is very important, it’s how your audience reacts to your emails. 

Let’s make it simple: Your recipients have agreed to receive your emails in a clear and explicit way. You’re providing them messages that they think are valuable, so it’s pretty clear that a large part of your audience will engage with your emails –  i.e., opening them or clicking through them.  This gives a very clear signal to the mailbox providers that your recipients want your emails. 

From this perspective, it wouldn’t make much sense for these same mailbox providers to start putting your emails in the spam folder or doing anything that would prevent your emails from reaching your audience.

Now let’s look at the same scenario, but with unwanted emails. Your audience receives them and doesn’t find them valuable enough to open them – or worse, they mark them as spam if they don’t remember ever agreeing to receive them.

If enough people from your audience do that, you’re now sending a very strong negative signal to the mailbox providers, informing them that your audience doesn’t find enough value in your communications. Those providers would then likely prevent more of your emails from reaching your audience – via putting them in spam or rejecting them entirely – in order to protect their users’ mailboxes.

So help the mailbox providers do the right thing when it comes to your emails. Send wanted, valuable content to your audience and reap the benefits of an engaged inbox. 

Outside of content creation, there are a lot of ways to improve engagement: implementing double opt-in, segmenting your audience, using clear unsubscribe buttons, and more. 

Embrace authentication 

If there is one topic in the email service industry that pretty much everyone agrees on: authentication is not optional 

Email authentication is the way to confirm to mailbox providers that you are who you say you are, and that your emails haven’t been tampered with in transit to your recipient mailboxes. In this day and age, it’s just good manners, really. It doesn’t take a long time to configure your SPF and DKIM records, and it shows that you know your basics in terms of email good practices. On top of that, it makes your email traffic more legitimate. 

Plus, authenticating your emails properly will be a great first step to adding a DMARC policy on your sending domain, and maybe even a BIMI record.

Be consistent

This one is probably hard to grasp, but it’s very relevant when it comes to the email traffic the mailbox providers are receiving every day. We’ve noticed above that they have to do a lot of work to fight spam messages or abuse, and ensure email security. Your emails are included in this work. Should they put them in the inbox or the spam folder? You need to send consistently to help them decide what to do – that’s how their spam filters work.

It doesn’t matter if you’re sending every single day, or every week, or two times a month. Because such sends are predictable from the internet service providers’ perspective. “Look, they’ve sent those past four campaigns once a week in the last month. We can expect the fifth one next week.” 

They will also consider the volumes sent each time. If you regularly send a weekly newsletter to 50k contacts, providers might not be too fussed if your list goes up to 55k subscribers one week. But what happens if you’ve been sending to 50k contacts over the last six months and suddenly increase your reach to 1M subscribers? Well, let’s just say that’s going to look a little suspicious. You need to ask yourself if your sending patterns can, and do make sense to the receiving side. 

Remember. The more predictable you are when sending your campaigns, the easier it will be for the email providers’ spam filters to adjust themselves correctly for your email traffic.

Do your due diligence with your lists

It was a best practice in the past, and it still is right now. Don’t send emails to invalid addresses. It’s that easy.

Again, imagine the receiving side. Mailbox providers have a lot of work to do to process all inbound email traffic. If you’re one of those people who adds invalid addresses in your lists, it means that you’re simply creating “noise” for those mailbox operators – they have to accept our server connections, simply to have them rejected right after because the mail cannot be delivered to the invalid address. By cleaning your lists before sending, you could easily avoid this situation. 

Using email validation software, or simply adding a double opt-in process to your newsletter flow, will prevent your lists from collecting invalid addresses.


Now you know: There are a lot of factors to consider when it comes to email deliverability. We'd love to have a quick answer to everyone's favorite question - How do I make sure my legitimate email doesn't land in the spam folder? - but the truth is... deliverability is complicated. Luckily, these tips will allow you to understand what mailbox providers care about and help you stay on the right path.

Be smart with your email traffic. Be a good sport and think about what the other side wants. Not only your audience, but also the mailbox providers actually servicing your subscribers. Think about what they expect from you, and not what you expect from your audience, or them. And just like people, if they like you, it’s likely that they’ll be very happy when they receive your emails. Talk about a win-win situation.

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