In email marketing, there are many battles you have to win to win the war.

But when people ask me what the most important item or skill to master is in order to improve the effectiveness of an existing email campaign, I tell them it has to be the subject line. While there are many different pieces of an email that have to come together to make it effective, messing up the subject line is the best way I know to fall flat on your face.

Here are my Top Five Mistakes not to make in your subject lines:

  1. Make it too long – This is a deadly mistake that too many companies still make. Long subject lines get cut off at different points, depending on how the recipient is reading their email. If your subject line gets cut off, it loses value. And with something as important as the subject line, loss of value is fatal. A good rule of thumb is 60 characters or less, including spaces.
  2. Don’t communicate value – This is the number one tip that I give to people when I teach. The subject line is the first chance you get to offer your value proposition. And the value in each email may vary, so you want to be sure that you don’t stray too far off your message. The question to ask yourself is, have I given the reader enough of a reason to open the email?
  3. Use the word “Free” – As I wrote in a guest post I did over at Ryan Stephens Marketing, the word “free” is one that will forever be flagged by spam filters. And while it may be tempting to offer Free Shipping, Free Trials, or Free Signups in your emails, it’s better to spell that out in the body copy than it is in the subject line.
  4. Too vague – I’ve seen this more recently, companies try to be cute and creative with their subject lines. In an attempt to keep it short, they say nothing. They tell you about a sale without telling you for what. Nothing says spam like a subject line I can’t understand.
  5. Use a dollar sign ($) – Back in October, I suggested 5 tests to try on your email marketing campaigns. And the first was the use of percentage vs. actual dollar amounts. The reason is that dollar signs are more likely to be caught by spam filters than percentage signs. There’s a reason Groupon shows you a percentage off every day and not a dollar amount off.

What are some mistakes that you’ve seen? Share your thoughts in the comments below.