We all know the level of effort that goes into getting an email campaign out the door—concepting, promo planning, writing, designing, coding—but once you hit the send button, where is your email going? Is it landing in your subscriber’s inbox in all of its glory? Or is it disappearing, hidden in their spam folder never to be seen? 

Seeing an otherwise flawlessly executed campaign land in the spam folder is an email marketer’s worst nightmare, and why deliverability should be top of mind at all times.

How is Deliverability Determined?

Different inbox providers (Gmail, Yahoo, Hotmail, etc.) use different algorithms to determine where your email goes once it’s handed off by your email service provider (ESP). 

Some key elements providers consider when deciding what to do with your email include:

1. Email Engagement

Providers look at email engagement as a key indicator of your email’s worthiness of the main inbox. If you’re emailing every user that has ever subscribed to your mailing list, chances are a lot of them aren’t opening your messages. Mailing regularly to unengaged contacts will eventually land you in the spam folder. Make sure you’re running unengaged contacts through a re-engagement series and removing dead contacts from your list regularly to avoid falling into this trap.

If you’re a high-frequency sender, and everyone on your list is being mailed every single campaign you send, you’re likely to see higher than normal spam complaint rates, which will similarly land you in the spam folder. Consider breaking your audience into frequency tracks based on engagement, mailing more frequently to those who open and click regularly, and pulling back the cadence for less engaged contacts.

2. Sending Domain

If you’ve had less than stellar sending practices in the past, your domain will have a tarnished reputation. On a similar token, if you’ve recently updated your sending domain, you’ll need to build up a positive reputation from scratch. To win favor from inbox providers in either scenario, run through a warming process—only mailing to highly engaged contacts until you see improvements—in order to build up credibility as a sender.


These are all the technical pieces that tell providers you are who you say you are. These authentication technologies acts as your company’s signature, confirming your status as a valid sender. Work with your IT team to ensure proper set-up.

4. IP Address

Much like the authentication factors above, a sending IP helps the inbox provider verify who is sending the message and whether or not they should be trusted. Many senders are on a shared IP address (especially those with small mailing lists or on free or inexpensive ESPs), meaning your reputation is tied up with those around you. Email providers are aware of the shared nature of many IPs, thus putting less weight on IP reputation, but a healthy IP is still an important factor in your overall deliverability mix. 

5. Content

Email clients have a long list of no-nos that can trigger a spam filter: Using large images, spammy words ( Free! Free! Free!), high image-to-text ratio, or high complaint/bounce rates all come into play when ISPs are determining where you’re going to land. 

It’s important to remember that inbox placement is a privilege, not a right, so following best practices when it comes to mailing and list hygiene will help prevent issues. 

Measuring Deliverability

Many marketers believe their ESP can provide a reliable view of their email program’s deliverability; just look at how many emails were ‘delivered’, or the sender’s ‘delivery rating’ as an indicator of whether or not emails are landing in the inbox. Unfortunately those numbers aren’t going to give you the complete picture. Your ESP’s reporting can only tell you whether or not the message was successfully handed off to the ISP’s servers. It can’t tell where it’s going once it reaches the ISP. 

So it’s time to take matters into your own hands. There are two key areas to look in order to determine inbox placement:

1. Track Open Rates by Domain 

A tanking open rate is the first indication that you’ve got a deliverability problem on your hands. If your emails are landing in the spam folder, chances are they’re not getting opened. 

Many ESPs offer out-of-the-box reporting for engagement metrics by domain. If your ESP doesn’t have this reporting, take a few minutes to create segments for the top 5-10 domains in your audience. Utilize these segments for regular deployments in order to track open rates by domain. In general, Gmail, Verizon (AOL and Yahoo domains), and Microsoft (Hotmail, Live, MSN, Passport, and  Outlook) will be the big ones to watch, and smaller domains can be bucketed together. This segmentation strategy will help you identify exactly which domains are having issues, enabling you to chart a path forward to get back into the inbox. 

2. Utilize Inbox Tracking Tools

For more detailed data on your inbox placement, consider contracting with a deliverability monitoring service like 250OK or ReturnPath. These vendors utilize seed lists (actual live email addresses) to determine exactly where your email is going, telling you not only if you landed in the inbox, but in the case of Gmail, helping identify if your messages are in the Primary or Promotions tab. While these helpful tools will allow you to catch wind of issues, they’re not always 100% accurate. Avoid unnecessary panic by monitoring open rates alongside for more realistic tracking. 

The sheer number of factors to consider when it comes to deliverability can be overwhelming. Take the first step toward long-term deliverability health by getting a clear view of current inboxing to help key into areas for improvement.

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