Email Deliverability and Efficiency

Updated August 28, 2023 | Mikhail Kornienko and Jane Herzig-Garza
Published January 23, 2017 | Mikhail Kornienko

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 beyond wizardry. There is, however, continual research on the topic. And there are some proven – dare we say? – rules which generally yield better results.  

Here are several big areas that need to be taken into account when building and sending out email campaigns: 

  • 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.” 

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 Email on Acid to test preview text across email clients in addition to proofing how emails render across browsers. 

For some emails, depending on how the content is organized, it could be difficult to have the best preview text possible, 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 play a huge role in the email even appearing 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. 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 ever reaching the user’s junk mail folder) 
  • In general, just write good, meaningful, friendly content — and this should be sufficient for deliverability 

Now, once we’ve made sure our email won’t be filtered out by robots (hopefully!), we can dig 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 2023, up to 78% 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 tags for accessibility. 
  • Make sure that all your “graphical” CTA buttons are still identifiable without actual graphics loaded. The best approach is to apply a contrasting background to a button’s table/cell/div. 
  • Oh, and just when you thought we were done with images – we aren’t! Make sure to compress the hell out of your images so they load as fast as possible (remember that 3/4+ of initial opens occur on mobile). At Cloudberry, we compress every single image manually. We use ImageOptim at an “insane” optimization level (with Guetzli compression algorithm enabled to compress JPEGs even further – though you should know that Guetzli only kicks in at compression levels of 84 and above), and when we need to optimize PNG images with alpha transparency, we use the free online tool Squoosh.  
  • Do consider the ever-improving displays of Apple and Android devices 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, Gmail, and Outlook web and mobile interfaces. As for many other email-related things, we use Email on Acid 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. 

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. 

SPF. 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 emails on behalf of that domain. 

DKIM. 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. 

DMARC. 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 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). 

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

Dark Mode Considerations 

One last bit of consideration must be given to the rise of Dark Mode on email platforms, such as Outlook, Gmail, and Apple Mail. Dark Mode inverts the colors displayed within email to reduce the eye strain on the user. While the merits and feedback on Dark Mode have been split, it is a reality that must be planned for when designing an email marketing campaign. But how? Should you have two entirely different versions to account for light and dark mode? The answer is maybe. While leaving email content in the light mode version may not create too many problems for those who have joined the dark side, it may not look as polished or legible, which could lead to decreased action taken by recipients and even open rates over time.  

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 number 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 have the time and the 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 open rates for emails are on Fridays 
  • Best click-through rate is for emails sent on Fridays 
  • Best response time is evening (although around lunch time local time works 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 the emails we’re sending. 

A friendly, longer 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, and 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: Cloudberry, Digital Marketing, Experience Design, Technology
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