The One Thing You Should Never Do In An Email Subject (Based on Data)

Posted By on May 9, 2017

Now and then, I get an email with a subject line in all capital letters, often “URGENT” or otherwise imploring me to read the message or take an action. I always wonder why people pen such subjects, as many email etiquette articles warn that writing in all caps can come across as yelling. But, netiquette aside, people do it anyway.

Might using all caps in a subject line be effective (even if rude?) Boomerang previously looked at how subject length affects response rate, but we’ll now look at whether or not uppercase subjects are a good idea if you want a reply (and then explore the data a bit further!)

Emails with All Caps Subjects Get Less Replies, Are More Likely to be Marked as Spam

We looked at over 300,000 publicly-available email threads and compared the response rate for emails whose subjects were in all caps compared to all others.1 The good news is that only half a percent of emails in our data set were written with an all caps subject line. The bad news (for the people behind such subjects) is that these emails had a markedly lower response rate compared other emails in the set:

Subject Type Response Rate # Emails
All Caps 34.6% 1,687
Others 50.0% 300,129

On average, users who wrote an email with an all uppercase subject received a reply 30% less often, relative to the other emails in the set.

Perhaps worse, however, is that emails with all caps subject lines are also less likely to even make it to someone’s Inbox. Some widely used spam detection systems look specifically for whether or not an email has a subject in all caps. If an email has such a subject, it serves as a strike against the email, and the message is more likely to be marked as spam.

It’s a bit ironic, but a critical email with a subject of “URGENT: PLEASE READ” could be less likely to make it to someone’s Inbox compared to an email with a less dire subject line. So if you know someone who loves their caps lock key, share this with them so they might reconsider their approach:

You should know that there are many factors beyond the case of your email’s subject that affect whether or not you’re going to get a response. Your email’s tone, length, reading grade level, and many other facets matter too. It can be a lot to juggle all of these things, but for anyone trying to improve their email’s chances for a response, there’s Respondable. Respondable is an AI-assistant that gives real-time feedback on how to craft the perfect email by looking at such characteristics. You can use the basic version for free with the Boomerang extension.

Add Respondable for GmailGet Respondable for Outlook

“But Really, Who Does That?”

So we’ve established that all caps subjects are bad, but who would send such an email? If you guessed AOL users, you guessed right!

Here’s how the likelihood of someone sending an email with an all uppercase subject fared, based on their email provider2:

The data shows that AOL users were nearly three times as likely to send an all uppercase subject compared to the dataset average: “YOU’VE GOT [ALL UPPERCASE] MAIL!” (But, to be fair, AOL messages only had an uppercase subject 0.6% of the time, even if they were the biggest offenders.) Conversely, messages from Gmail or Google Apps email addresses were the least likely to TYPE LIKE THIS, with such prose in just 0.1% of Gmail/Google Apps message subjects in our data set.

The data suggests that most people reading this are already practicing good email etiquette and not sending emails with all uppercase subjects. That said, many of us have that one colleague or uncle or friend who still sends uppercase-heavy emails from time to time. If you’ve always wanted to say something, but lacked a source to cite, we hope that you’ll share this post and get them on the path to writing more effective emails! (And if you want to hear about more ways to make your emails more effective, subscribe to our blog to get our latest data studies in your Inbox!)

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Lastly, 100% of the data used for this analysis was from public email archives provided by various online communities, ranging from an instant messaging client’s support emails to various open source software communities to UCLA’s religion law listserv. We used an open-source tool called BigBang to collect emails from these and other public archives.


  1. An “all caps subject” was defined as a subject that contained no lowercase letters, but had at least three letters. We likewise only used subjects with at least three letters for “other” subjects as well.
  2. Gmail category also includes Google Apps users. Here’s the math for this graphic: (% of emails w/ all uppercase subjects sent by provider – % of all emails w/ uppercase subjects) / (% of all emails w/ uppercase subjects). We should also mention that our sample size wasn’t huge, at about 1700 all-caps subject emails, so we wouldn’t recommend basing your PhD thesis on these numbers.

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