What is deliverability?

‘Deliverability’ refers to the likelihood of your emails landing in subscribers’ inboxes.

If your campaign has high deliverability, almost every email will end up exactly where you want it (in inboxes)

If your campaign has low deliverability, many of your emails will have bounced, been blocked, or otherwise failed to reach the inbox.

Deliverability is largely based on your ‘Sender Reputation’. This is a score assigned by ISPs, based on how valuable, engaging, and generally, desirable your emails are to inbox users.


Why is it important to monitor deliverability?

This may sound obvious – but if your emails aren’t reaching the inbox, they won’t get read.

It doesn’t matter how brilliantly you’ve crafted your campaigns, all your hard work will go to waste if deliverability is low.

Keeping an eye on your deliverability ensures that you’ll know straight away when something goes wrong. You can quickly jump in to fix any issues, and ensure that every campaign gets in front of the right eyeballs.


How can you monitor deliverability?

We went into detail on metrics in our last blog so we won’t get too deep into it this time! If you want to know more about each metric mentioned here, what it measures, how to measure it, and so on, check out the previous blog:

For deliverability, the metrics you need to focus on are:

  • Hard bounce rate. This one is particularly important. Hard bounces will crash your sender reputation in no time.
  • Soft bounce rate. This one isn’t as urgent as the hard bounce rate, but it’s still worth keeping an eye on. Too many soft bounces aren’t great for your sender reputation – so if an address is frequently soft-bouncing, it’s probably worth either deleting them or putting them on an exclusion list.
  • Spam/complaint rate.
  • Engagement in general. Falling engagement often comes just before a drop in deliverability, so watch your engagement metrics like a hawk!

What affects deliverability?

Deliverability is controlled by ISPs (Internet Service Provider e.g. Gmail). The job of the ISPs is to keep the inbox a safe and interesting place for their users.

In order to do this, they keep a close eye on sender behaviour. If a sender consistently has high engagement rates and low spam/bounce rates, the ISPs will infer that customers are enjoying their content. That sender will enjoy a high sender reputation.

However, if a sender’s emails are bouncing, getting flagged as spam, or even simply being ignored, the ISPs will act to protect their customers. They will assign a low reputation to this sender, and deliverability will drop.

As a general rule, ISPs value senders who are:

  • Trustworthy
  • Consistent
  • Engaging
  • Reliable
  • Not on a blacklist (more on blacklists in a bit!)

So far, so simple, right? Well, ISPs also employ ‘spam traps’ which make things a bit more complicated.

Spam traps

A ‘spam trap’ is an email address that ISPs use to catch unscrupulous senders.

Broadly speaking, there are two main types of spam trap to look out for:

  • Pure spam traps. These are addresses created for the sole purpose of catching bad senders. Because these addresses have never been owned or used by a real-life human, the only way a sender can end up with a pure spam trap in their list is by harvesting them illegitimately.If you bought, scraped, or otherwise obtained an email list without permission, chances are that it will contain pure spam traps. If you then send emails to those spam traps, your sender reputation will tank.
  • Recycled spam traps. These are addresses that have become inactive, and been repurposed by ISPs as spam traps. If you have a recycled spam trap in your list the ISP will assume that you aren’t regularly checking and cleaning your list – which, from the ISP’s perspective, isn’t great.

To avoid stumbling into spam traps, always get email addresses legitimately (with permission!), and keep your list nice and clean.



If you’ve been a bad sender in the past, you may be on a blacklist. Blacklists are exactly what they sound like – lists of bad senders. If your IP address appears on a blacklist, the ISPs will be very wary of you.

You can check your blacklist status with tools like MXtoolbox. If it turns out you are on a blacklist, a message to the list’s owner should sort things out.


Send volumes

So, to recap, deliverability is affected by:

  • Engagement rates
  • Spam traps
  • Blacklists
  • Spam/complaint rates

Is that all? Not quite. ISPs also value consistency.

If your send volumes suddenly increase, the ISPs will get suspicious. And, when ISPs get suspicious, your sender reputation usually suffers. So, if you want to increase send volumes, it’s best to do so gradually.

‘Warm up’ your campaign by slowly increasing send volumes over time. If you find your deliverability dropping off, slow it down again.

Deliverability is largely based on your ‘Sender Reputation’. This is a score assigned by ISPs, based on how valuable, engaging, and generally, desirable your emails are to inbox users.

Deliverability tools and techniques

If your client is struggling to reach the inbox, there are a few tools and tricks that might help:

  • We’ve mentioned this above. MXToolbox will tell you if you’re on any blacklists.
  • Authentication protocols. Adding authentication protocols like encryption, SPF, DKIM, DMARC, and BIMI will all give your client’s emails greater legitimacy. Secure authentication is a great signal to the ISPs that you are serious about email and dedicated to your customers’ safety.
  • New IPs. If your send volumes are increasing and you’re struggling to throttle campaigns through one IP – add another one! A new IP will need careful ‘warming’ (i.e. increasing send volumes a little at a time), but eventually it could work wonders for overall deliverability.
  • Great content! High engagement is key for high deliverability. If engagement is falling, it might be worth testing out content elements like subject lines, CTAs, and so on. Simply optimizing your clients’ content could work wonders.
  • MailTester is a simple way to analyze key elements of your clients’ emails. It will show you what’s well-configured, and what needs work. The better the content, the higher the chances of getting reputation-boosting engagement.
  • SenderScore will tell you your sender reputation, on a scale of 1-100. Generally speaking, a score under 70 needs some work.

Deliverability – the key to the inbox

Your campaign can’t succeed if nobody sees it! So, keep a close eye on your deliverability. If engagement starts dropping and bounce rates start rising, it’s time to lift the hood and fix your sender reputation.

It’s usually not hard to fix problems with a sender reputation – but you won’t know what needs fixing if you’re not keeping a close eye on it! So, make sure your clients’ campaigns always get priority in the inbox by paying attention to deliverability.

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