With groaning inboxes, is it any wonder why so many e-mail messages get ignored by their recipients?

Indeed, with it costing so little to send an e-mail – especially when compared to the “bad old days” of postal mail – it’s too irresistible for marketers and others to deploy hundreds or thousands of e-mail missives at a pop, even if the resulting engagement levels are so paltry.

And therein lies the problem: The “value” of such e-mails diminish to the point where recipients have a very good idea of their (lack of) worthiness without needing to open them.

In such an environment, what’s the the likelihood of something important inadvertently slipping through the cracks? Not so great.  And so users go on their merry way, hitting the delete key with abandon.

Faced with these realities, anything senders can do to improve the odds of their e-mails being opened is worth considering.

As it turns out, some of those odds can be improved by focusing on the e-mail’s subject line.

We know this from research conducted recently by e-mail platform provider Yesware. As reported this week in Fast Company, Yesware’s data scientists took a look at ~115 million e-mails of all kinds, gathered over the course of a 12-month period, to see how open rate dynamics might be affected positively or negatively by differences in the subject line.

The Yesware analysis was carried by analyzing most- and least-used words and formats to determine which ones appeared to be more effective at “juicing” open rates.

As the benchmark, the overall e-mail open rate observed across all 115 million e-mails was 51.9% and the overall reply rate was about 29.8%. But underneath those averages are some differences that can be useful for marketers as they consider how to construct different subject lines for better impact and recipient engagement.

The findings from Yesware’s subject line analysis point to several practices that should be avoided:

Subject line personalization actually works against e-mail engagement.

It may seem counterintuitive, but adding personalization to an e-mail subject turns out to suppress the open rate from 51.9% to 48.1% — and the reply rate goes down even more dramatically from 29.8% to 21.2%.

Yesware surmises that this seemingly clever but now overused technique bears telltale signs of a sales solicitation. No one likes to be fooled for long … and every time one of these “personalized” missives hits the inbox, the recipient likely recalls the very first time he or she expected to open a personal e-mail based on such a subject line – only to be duped.

“First time, shame on you; second time, shame on me.”

Turning your subject line into a question … is a questionable practice.

Using a question mark in a subject line may seem like a good way to add extra curiosity or interest to an e-mail, but it turns out to be a significant turnoff for many recipients. In fact, Yesware found that when a question mark is used in the subject line, the open rate drops a full 10 percentage points (from 51.9% to 41.6%) – and the reply rate also craters (dropping to 18.4%).

It may be that turning a subject line into a question has the effect of reducing the power of the message. Yesware data engineer Anna Holschuh notes that posing a question is “asking a lot of an already-busy, stressed-out professional.  You’re asking them to do work without providing value up front.”

On the other hand, two subject line practices have been shown to improve e-mail open rates – at least to a degree:

Include numbers in the subject line.

Subject lines that contain “hard” numbers appear to improve the e-mail open rate slightly. Yesware found that open rates in such cases were 53.2% compared to 51.9% and the reply rate improved as well (to 32%).  Using precise numbers – the more specific the better – can add an extra measure of credibility to the e-mail, which is a plus in today’s data-rich environment.

Use title case rather than sentence case.

Similarly, Yesware has found that the “authority” conveyed by using title case (initial caps on the key words) in e-mail subject lines helps them perform better than when using the more informal sentence case structure.

The difference? Open rates that have title case subject lines came in at 54.3%, whereas when using sentence case in the subject line resulted in open rates at just 47.6%.

Similarly, reply rates were 32.3% for e-mails with subject lines using title case compared to 25.7% for e-mails where the subject line was sentence case — an even more substantial difference.

Generally speaking, e-mail marketing succeeds or fails at the margins, which is why it’s so important to “calibrate” things like subject lines for maximum advantage. The Yesware analysis demonstrates how those tweaks can add up to measurable performance improvements.