april 11, 2023

Why Delivery Volume is a Great Target Metric for Email & SMS Campaigns

When we think about campaign planning each month, we like using message delivery volume as a target metric.

Most people tend to make the mistake of focusing on the number of campaigns per month or week, rather than the actual number of deliveries.

We like to invert this paradigm and instead start with the target amount of deliveries, then back out the number of campaigns to specific target segments needed in order to hit the delivery volume amount.

The goal here is to predictably match and exceed previous campaign revenue results each month (adjusted for factors like seasonality, inventory shortages, etc of course).

The general thinking here is based on the idea:

Revenue = Rev/Delivery x Volume of Deliveries

Given a fairly consistent rev/delivery ratio, your main revenue lever is that volume of deliveries variable.

This idea can be applied more generally at the account or channel (email, SMS) level or better yet, drilled down to specific segment/theme. A snapshot example:
When it comes to email and SMS campaigns, a volume baseline generally needs to be met to generate, at least, similar results to the previous month.

If you fall significantly short on the volume of deliveries, your conversion rates need to work extremely hard to make up the difference.

For example, if last month you had 400,000 total campaign deliveries and this month you ended up with half of that (200,000), then just to stay even month-over-month on campaign revenue, you'd have to have double the sales conversion rate (or AOV).

Driving up sales conversion rates is certainly possible over time, especially with effort put into testing/segmentation and also automated flows. But to double campaign sales conversions immediately across the whole set of campaigns is rarely a one-month job. And even then, in this scenario, you'd just be staying even.

By using volume of deliveries as a parameter target, you make sure you are planning enough campaign frequency to enough of the list so that the likelihood of hitting campaign revenue targets is high.

A side benefit is you can also keep better tabs on your expected ESP costs and scale up or down accordingly, as most charge based on volumes of deliveries. This comes into play even more with SMS or MMS deliveries, which tend to have a higher cost-per-sent message attached to them compared to email.

Hopefully you're convinced of the importance of using email delivery volume as a core target to structure your campaign planning around, let's dive into the nuances of how to back out the exact volume targets.

Diminishing Marginal Returns per Delivery

All else being equal, if you keep seeing great trends in said metrics, you can keep increasing the volume of deliveries. However, there inevitably will reach a point of diminishing returns, at which point you'll need to dial back delivery volume increase and reassess relative to the changes in the metrics.
For example, here you can see good initial growth relative to increasing deliveries, but the revenue curve eventually starts tapering off as the diminishing returns start kicking in.

A ratio view of this would show:
Where the per unit revenue effectiveness of increasing deliveries is decreasing over time as the number of deliveries ramp up.

At this point, purely increasing deliveries through larger volume target segments or higher campaign frequency will have relatively smaller returns. In these cases, it makes sense to keep deliveries more steady instead of continuing to increase them, and seek to improve conversion rates across the regular cast of campaigns and flows to continue to gain more revenue out of the existing base.

If you have historical email campaign data to work with, this is a good place to start. Namely, look back and plot your deliveries vs. revenue.

If you generally are landing on the upswing of the curve where the historical months of higher deliveries are matched with a corresponding lift in revenue, then you are most likely below your optimal delivery volumes.

On the other hand if you see a significant flattening out of the revenue curve relative to increases in deliveries, you are likely at or exceeding your optimal delivery volumes.*

*Please see adjustments for list attrition and seasonality as discussed in below sections.

If you likely haven't exceeded your optimal delivery volumes yet, one way to plan the target volumes is to take something like:

Campaign Delivery Target = List Size x Number of Campaigns x 45%

The percentage in this equation is an adjustment for the average portion of your full list you target with a given email campaign and very likely to vary depending on your specific setup. This assumes you are at least doing some segmentation for each campaign, such as engagement segmentation where you are adjusting frequency of campaigns based on subscriber engagement levels. Something like:

  • Highly Engaged: receives 100% of all campaigns
  • Medium Engaged: receives 50% of all campaigns
  • Low Engaged: receives <25% of all campaigns

Combining these delivery volume parameters with the theme-based approach suggested in my other post, you may end up with something like this:
List Attrition

Losing subscribers, i.e. list attrition, is a natural part of the subscriber lifecycle. Any email or SMS subscriber list that is regularly sent to will see some list attrition in the form of unsubscribes, bounces, and spam complaints. The key is you want to maintain sustained, net positive list growth after accounting for list attrition.

So ideally as you increase email deliveries…
You still see sustained positive net list growth.
Increasing campaign delivery volume will at some point start adversely affecting list attrition by increasing the rate of losing subscribers. Depending on the corresponding subscriber list acquisition efforts, you could be looking at list growth dipping into the net negative territory.

As a general rule, we want to avoid sustained net negative list growth as it erodes the total effectiveness of your future campaign efforts over time. A good target to shoot for is 7% net list growth per month.

As average list attrition is over 20% per year, or 2% per month, this would mean nearly a 10% gross increase. Less than 3% net generally means you are probably not staying ahead of the people who are stagnating, but haven't unsubscribed.


If your product lines are highly seasonal, you'll want to adjust your delivery volume targets accordingly.

During low season:
Follow a similar approach as already outlined, but a lower "maintenance" frequency of campaigns and delivery volume. Here the goal is to keep the subscriber list warm (which also helps your deliverability in high season), optimize to minimize subscriber churn, and to generate a consistent low-season revenue level. In general, the metrics of success during the low season are:

  • Keep Open/Click Rates steady and a decent absolute level
  • Minimize unsubscribe rates and being very mindful of a net positive list growth (or at least net neutral)
  • Maintaining email as a steady percentage of monthly revenue is successful. Any increases here should primarily come from adding or optimizing for efficient campaign/flow emails rather than large increases in delivery volume.

Low season is also a great time to work on optimizing flow funnels and segmentation, particularly along user conversion paths like:

  • Visitor > Lead
  • Lead > First time customer
  • First time customer > Repeat customer
Better efficiencies here will really start to show once you ramp up volumes going into high season.

During high season:
High season brings an influx of demand and visitors. You are probably ramping up acquisition spend on traffic as well, so the email/SMS retention channels should also kick up a notch in terms of delivery volumes.

You'll certainly want to increase campaign frequency, alongside the inherent flow volume increases. Depending on the degree of your seasonality, you could be planning on multiple times higher campaign delivery volumes.
You may even dip into slightly negative net list growth in this period in order to maximize the revenue potential of the high season demand.

We calculate a seasonality index as a type of multiplier for what the ideal delivery volume is from month to month.
Campaign vs. Flow Volumes

So far the discussion on delivery volumes has mostly centered around email campaigns. This is rightfully so since campaigns tend to account for the vast majority of total delivery volume, and much more subject to big swings just by adjusting target segmentation and campaign frequency.

That said, flows (automations) also follow a similar principle and can be planned out to achieve optimal delivery volume targets.

Flow email volumes tend to be more stable than email campaign volumes. They do experience increased fluctuation when:

  • Entire new flows are introduced
  • Additional flow touch-points are added or subtracted
  • Volume of users to the email list/site increases
  • Filters and triggering criteria are changed
Even though flows have a different mechanism than campaigns, you should still evaluate each flow touch-point as a discrete message relative to campaign messages.

Since flows generally have much higher revenue per delivery compared to campaigns, when you see individual flow messages underperforming your top campaigns, you should consider culling those messages and/or refreshing them to bring performance back up.

Flows also tend to have diminishing returns; the longer the flow, the less productive the latter touch-points tend to be. For example, even in a short welcome series, you can see a steep drop-off in revenue per delivery:
And usually a more steady drop in engagement:
In these cases of specific under-performing flow touch-points, it's better to expose those subscribers to your campaign message rather than the corresponding flow message.

Conversely, if a flow continues to perform very well relative to campaigns, you should consider increasing the volume via adding additional touch-points or adjusting the filter logic to open up the funnel to more volume.

It's important to remember when calculating flow volumes is to start by factoring in flow deliveries and then adding the additional campaign delivery volume. You don't want to find yourself at the end of the month not realizing that you've used up all of your bandwidth and your flows don't go out.

A small additional note: if you have some top-performing "evergreen" campaigns, you should consider recycling the content of those messages and rolling them into a flow. This way the newer subscribers on your list can get exposure to this proven effective message, and you'll generate a higher revenue per delivery as a result.

In summary, delivery volume is a wonderful reference metric for email campaigns and flows. By setting appropriate delivery volume targets each month for your email marketing messages, you can better ensure consistent growth and hitting of revenue targets. I highly recommend including this metric into your email content calendar planning.

Want some guidance on how to strategically plan your email campaign calendar? Let's set up a call and discuss.

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