Email Deliverability and Efficiency

Even with all the advancements in content delivery media, email still stands as one of the most widely used and trusted approaches for targeted content delivery.

However, the art of delivering email to a recipient’s mailbox nowadays is pretty much a black magic art. There is, however, continual research on the topic. And there are some proven – dare we say? – rules which generally yield better results.

So, anyway, here are some things we at Cloudberry consider when building and sending out email campaigns.

There are several big areas which need to be taken into account:

  • Email authoring
  • Email delivery setup
  • Email delivery strategies

Email authoring

How an email is authored from content, to design, to under-the-hood coding, has a huge impact on whether it will be opened and read by the intended recipient. Or even delivered to the recipient’s inbox at all. There are a few strategies that will increase your chances for both.

Content is king

Sure enough, content is what keeps email together and what makes or breaks everything. Plan your content carefully. Below are a few things to consider:

Email subject line

  • Keep the email subject line ultra-specific and make sure it clearly communicates the email’s intent and/or content
  • Keep the subject line length within 40 characters long (28-39 characters gets the best opens)
  • Don’t use spammy subjects (all caps, spammy words, marketing fluff)
  • Having personalized subject lines does not help much (even worse, it could actually have negative effects)

Preview text

Every email client displays a small chunk of preview text within the messages listing. This is called “preview text” (more info on Litmus).

Preview text in Mail app on iPhone

Preview text in Mail app on iPhone

Make sure you keep the preview text length around 100 characters, and try to keep the core message within the first 50 characters. At Cloudberry, we use Litmus to test preview text across email clients in addition to proofing how emails render across browsers.

For some emails, depending on how content is organized, it could be difficult to have the best preview text possible just based on the flow of the content. In that case, it is possible to use hidden text, which will be rendered in the email list preview, but won’t be present when the recipient actually opens the email. Below is the code that can be used:

<div style="display:none;font-size:1px;color:#363839;line-height:1px;max-height:0px;max-width:0px;opacity:0;overflow:hidden;mso-hide:all;">
 Put your preview text here. Place it right after the opening BODY tag.


Email content

And here comes the king. But we won’t be talking about the actual meaningful content here. Rather, we will touch on the technical aspects, which plays a huge role for the email to even appear in your user’s inbox (vs. being filtered as junk or completely killed off by server-side spam filters).

Before email is even considered for delivery to the end-user, it is tested by the email server’s set of filters. These scan through email content and set a “trustworthiness” score of the email. In many cases, the results of the scan are added to the email in a way that is only visible to the email client (which, in turn, decides what to do with a particular email, based on these scores, as well as their internal algorithms).

This means that email content should be very carefully crafted, so that the email looks acceptable to the robotic filters first.

Some things to consider:

  • Don’t use ALL CAPS text at all if possible, but, if you do, keep it to a minimum
  • Don’t use MiXeD CaSe text
  • Don’t use frequent changes in content font size and color
  • Avoid general spammy words
  • Avoid an excessive number of links inside the email
  • Even if your email turns out to be mostly graphical, make sure it has a sufficient amount of text content. The reason is that spam filters can figure out the images-to-text ratio of emails, and the ones with a high ratio of images have a very high probability of being filtered out as spam (often on the server-side without even reaching the user’s junk mail folder)
  • In general, just write good, meaningful, friendly content — and this should be sufficient for deliverability.

Mail filters

Now, once we’ve made sure our email won’t be filtered out by robots (hopefully!), then let’s get deeper into the content itself. At this point, the email has reached the final recipient’s inbox. But will it be read? And acted on?

Again, we’re going to skip the copy-related things and outline the technical ones. Read on!

  • As of 2016, more than half (around 62% according to Litmus) of initial email opens happen on mobile devices, with iPhones leading the charge. So, make sure that either your email is responsive OR that even if scaled-to-fit on a mobile device, it is still readable and makes sense.
  • Think of users who might have an initial look at your emails without any images loaded. This can easily happen based on a user’s email client configuration and the email’s trustworthiness level. Make sure the email still makes sense without images.
  • For all images, except strictly decorative ones, make sure to include the ALT tag.
  • Make sure that all of your “graphical” CTA buttons are still identifiable without actual graphics loaded. The best approach is to apply a contrast background to a button’s table/cell/div.
  • Oh, and just when you thought we were done with images – we aren’t, yet! Make sure to compress the hell out of your images so they load as fast as possible (remember that 50%+ of initial opens occur on mobile). At Cloudberry, we compress each single image manually. Our toolset is: ImageAlpha, followed by ImageOptim at “insane” optimization level.
  • Do consider retina displays (30%+ of email opens are on retina iPhones, for example) and include 2x assets instead of 1x. These often don’t add much to file sizes, but give a much better initial impression to users with these displays (and no issue to users without). At Cloudberry, we only include 2x images, no 1x at all (unless they DO add a significant amount to download size, of course).
  • Adding personalization to your email on the content body level actually is not a bad idea.  If you have the data to actually personalize the email, it might make sense to do just that.
  • TEST! TEST! TEST! Make sure your email renders as perfectly as possible across all major email clients. The minimal set would be: iOS Mail, Outlook 2007+, Gmail, Office 365 and Yahoo web interfaces, as well as Android. As for many other email-related things, we use Litmus to perform our tests.

That’s a lot to consider, but the above should give a good boost to your email efficiency.

Now, let’s talk about some technical aspects of deliverability.

spam can

Delivery setup

Besides server-side and client-side content analysis, there are actually more “delivery safety” tiers that exist, and we will cover the three major ones.

Basically, what these do is assist in building trust between email servers themselves.


An abbreviation for Sender Policy Framework. This mechanism makes sure that an email from a particular domain comes from email servers that are authorized to send email on behalf of that domain.


That’s an abbreviation for DomainKeys Identified Mail. This mechanism aims at spoofing prevention, making sure that if an email message claims to be coming from a specific domain it does, indeed, come from that domain.


Or Domain-based Message Authentication, Reporting and Conformance. The goal of this mechanism is, again, to prevent spoofing and fight phishing and email spam. It is a relatively new technology, but by now is widely deployed on high-volume email providers.

Without going in too deep just make sure you have at least SPF and DKIM setup for your own email server (be it your rental/dedicated email server or your cloud mailer such as Office 365 for Business). If you’re using bulk mail services like Campaign Monitor make sure you authorize them to send emails on behalf of your domain (see instructions for Campaign Monitor at this link).

In case you’re using public email services (Gmail, Yahoo,, you’re covered and there’s nothing you need to do (or can do, really…).

Delivery strategies

So now we have a perfect email and a perfect setup. We’re definitely good to go, right?

Say “hello” to the human factor!

With the large amount of emails many users are receiving daily, there’s only so much attention and time they can devote to reading them. We can’t really force people to read our emails, BUT we can try to get through to them, when they are more likely to actually have time and willpower to pay attention.

There are several research findings/statistical data, which, when their wisdom is distilled and compressed, result in the following:

  • Best ROI is for emails sent on Mondays
  • Best click-through rate is for emails sent on Fridays
  • Best response time is evening (although around lunch time local time works OK as well)

This surely is not a magic bullet, but, thinking logically, the above just serves as a meaningful baseline. And once we have this baseline logic, we can work off of that depending on the nature of emails we’re sending.

A friendly long-ish newsletter? Well, Saturday late evening might work pretty well! Something that needs consideration and willpower? Hm… Monday? Around lunch-time? Who knows! Just might work, right? Think carefully, follow meaningful rules, but don’t forget that being “brave” and breaking patterns can sometimes work wonders! So experiment, iterate and try to find your best approach.

Good luck with your campaigns! And be a good netizen!

Categories: Digital Marketing, Technology
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