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To get a full understanding of the various metrics you can use to measure the performance your email campaigns, you need to start with knowing how many emails you have on your database, how many emails have been sent and how many have actually been received by your subscribers. As well as giving you a sense of scale, these form the basis of all other metrics used by email marketers.
Mailable list size
Total number of addresses you have permission
to send marketing emails to
This definition will give you the true size of your email database. Comparison between this and the total number of addresses in your database is an indication of the effectiveness of your opt in procedures. Email marketers should do everything within their power to grow this list over time, because email is the cheapest way of getting a message to your customers and prospects. New additions to an email list are also more likely to interact with your messages as they have recently expressed an interest in your brand.
Attrition, combined with insufficient list growth will result in a decreasing mailable list size over time. Some attrition is inevitable, (see unsubscribe rate) but should be offset by ensuring sufficient resource is invested in list growth (see subscription process).
When tracking metrics over a period of time, it is easy to forget the details of each campaign. We recommend recording as many details as possible for future comparison. Information such as date sent, time sent, subject line, creative/template used, segmentation/list name, list size etc. will prove invaluable when you are trying to identify causes of spikes or troughs in other metrics.
One easy way to save this data is alongside your sent data. If you can use your ESP's platform for historical data, it is imperative that you follow good file naming conventions and download the data periodically as a back up and before archiving occurs.
Sent/Pushed
Total number of emails sent
This will tell you exactly how many email addresses you sent any given message to. Apart from your mailable list size, this is the only number that's usually reported. These numbers don't lie. They represent the scale of your campaigns and give context to all percentage figures reported.
The total number of emails sent may be the same as your mailable list size, or the size of the segment selected.
Delivery rate
Number of delivered messages
Number of sent messages
(expressed as a %)
The number of emails delivered compared is like the number of people who finish the London marathon, as opposed to the number who started. Any email address that is not reported as delivered is defined as a soft or a hard bounce.
Ultimately, your delivery rate will be affected by your reputation. A high delivery rate is a result of ensuring you (and anyone sharing your IP address) following best practice in data acquisition and management, along with appropriate authentication, white-listing and accreditation.
Don't automatically assume, however, that because your message is reported as delivered, that it has reached your subscriber's inboxes. Delivered messages may end up in a junk folder or may occasionally fall into an IT black hole, where an ISP has neither delivered the message, nor reported it as a bounce. Following best practice and encouraging subscribers to add you to their address book will increase your chances of being delivered to your subscriber's inboxes.
For more information of reputation, watch Talking Email: deliverability and reputation.
Hard bounce rate
Number of hard bounced messages
Number of sent messages
(expressed as a %)
A hard bounce is caused by a non-existent, invalid or expired email account, or by syntax error in the address. This metric in an indication of the integrity of your data collection processes. A hard bounce rate that's consistently high or increasing over time suggests a pressing need to investigate your data collection processes or poor list management.
Hard bounces should be removed or suppressed immediately. Failure to do so will negatively affect your delivery rate as ISP monitor levels of hard bounces to assess your reputation.
Soft bounce rate
Number of soft bounced messages
Number of sent messages
(expressed as a %)
Soft bounces are defined as temporary delivery failures. Historically soft bounces have not been seen to be a major problem. In fact, almost every definition of soft bounces leads with 'inbox full' leaving most people the impression that it was a temporary problem and out of your control. Best practice also states that addresses should be removed after three consecutive bounces, further reducing your sense of urgency.
These views have not been updated for at least ten years, however, here at Alchemy Worx we believe that soft bounces should be treated very seriously indeed. Inbox sizes have dramatically increased over the years so 'mailbox full' messages are highly unlikely to occur, so the most likely cause of soft bounces are caused by ISP blocking.
Over the years, the way ISPs deal with spam has become incredibly sophisticated and they are very quick to start blocking emails when they identify suspicious patterns of behaviour from a given sender, IP address or range. Emails blocked in this way are recorded as soft bounces, so your soft bounces are important indicators of poor delivery practices.
Typical causes of blocking are: lack of authentication or accreditation; poor or unproven IP reputation; fluctuations in the volume of messages sent from the IP address you are using and spam complaints attributed to your IP address.
It's important to bear in mind that you may incur soft bounces because of the behaviour of companies sharing your IP address, making the way your ESP handles its shared IP addresses very important. For more on this topic, see Talking Email: deliverability and reputation.
ESPs will typically re-try soft bounces over hours or days, so this rate can go down after a message is deployed. Nevertheless, soft bounces should be constantly tracked and monitored by domain in order to determine where any issues are arising.
Unsubscribe rate
Unique number of unsubscribers
Number of delivered messages
(expressed as a %)
The unsubscribe rate is the proportion of subscribers that received your email and requested to be removed from your database. To keep this rate low, you need to make sure you follow best practices around data collection and maintenance, provide sufficient accurate information at the registration stage, and populate your communication with relevant, value-added content.
Certain industries and sectors can have inherently high unsubscribe rates based on the lifecycle of a product/service For instance, a subscriber who signs up to receive daily rental properties from a property website will be likely to unsubscribe when they have found a property. In such a situation, it's vital to pay particular attention to making the unsubscription process fast and easy. Overall trends should be monitored, rather than actual values.
Whilst it may be tempting to reduce attrition by making it difficult to unsubscribe, the consequences have a big impact on deliverability since, without the option to unsubscribe easily, many subscribers will use the "report SPAM" button to unsubscribe, which will seriously affect your deliverability. A low unsubscribe rate, combined with a high level of complaints, or low delivery rate would indicate that your subscribers find it difficult to unsubscribe, and you should rethink your unsubscription process.
Complaint rate
Number of complaints received
Number of messages sent via FBL domains
(expressed as a %)
The number of complaints you receive can greatly affect whether your emails are delivered to your subscribers inboxes. ISPs use the number of complaints received to decide whether emails are coming from legitimate marketers or spammers. Not all ISPs have feedback loops so the complaint rate is expressed as a proportion of messages sent to the domains that you are monitoring for complaints. Whilst you cannot then apply this proportion to the remainder of your database with any accuracy, it does give you an indication of the level of complaints you are receiving.
A high complaint rate indicates that you are not following best practice in your email marketing overall. You should ensure that your data acquisition practices are clear and honest, and that too much time does not pass between when a subscriber registers and they receive their first mailing, or between regular mailings. Recipients who were not expecting an email from your company and do not recall signing up to receive communication from you are likely to report your message as SPAM.
If you have a high complaint rate, combined with a low unsubscribe rate you need to assess your unsubscription process as your subscribers may be finding the process difficult or cumbersome. If a subscriber cannot easily unsubscribe, they may simply use the "report SPAM" button to do what is effectively the same outcome for them.
Finally, ensuring your content is valuable and relevant to your subscribers will result in recipients not viewing your message as SPAM protecting both your reputation and your deliverability.
Almost all ESP's and most industry benchmarks use the number of messages delivered rather than the number sent as the base of interaction metrics. Don't miss next month's Email-Worx Right Brain for more about why this is so, and how it can affect you...
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