Which DOCTYPE Should You Use in your Email Marketing Templates?

I’ve been doing a bit of email-related development at work lately, writing code that would allow us to switch our high volume email stream from an old and costly Email Service Provider (ESP) over to a newer and more affordable ESP on a template-by-template basis. Slowly increasing your email volume on new IP addresses is an important aspect of Email Marketing known as “warm up”.

As we were moving our email templates over to our new ESP, we also updated some of the older ones that hadn’t been touched in many years to implement our mobile-friendly layout.

One question that came to my mind while updating these templates was which DOCTYPE we should be using. My own gut instinct was that we should use the DOCTYPE that most closely describes the markup of our email template; in other words, the DOCTYPE that our HTML would best validate under.

However, I also thought it would be a good idea to look at what the large volume senders are doing, including the ESPs themselves, to see if there was any kind of consensus on the best DOCTYPE to use.

I decided that the best way to look for a consensus was to simply take a look at 25 random emails from large ESPs, Email Marketers, Social Media companies, Retailers, and others that were in my very own inbox.

DOCTYPE Results

First, let’s take a look at a table containing the raw data that was produced.

Chart of DOCTYPE Data

Analysis

It certainly doesn’t appear that there is any consensus on the best DOCTYPE to use among the top companies that I receive email from. What I found particularly interesting is that even departments within the same company had chosen different DOCTYPEs – this was the case for both Amazon and Google.

In this admittedly small sample of emails, HTML 5 and XHTML 1.0 Transitional tied for the most popular DOCTYPE, each representing 8 out of the 25 total email messages that were analyzed.

Conclusion

I feel like the fact that this data showed a variety of DOCTYPEs in-use among the largest of mailers does back up my initial instinct that any of the popular DOCTYPEs are fine to use so long as your markup validates using the type that you declared.

An interesting followup might be to check whether mailers are in fact sending out emails with markup that validates, or whether they are able to reach the inbox simply due to their size, mailing history, and user engagement metrics.