What does a good subject line consist of?

A good subject line is your one-way ticket to maximise your open rates. Before any interaction and conversion, your contacts will use your sender email address and name and subject line alone to decide whether they're going to open your email - or move it to the trash. 

A creative or fun, but most importantly clear subject can make the difference between your emails being an annoyance or a delight when they pop up in your Contact’s inbox.  

A few tricks to try

Your subjects should be tailored to your business, your intent, and your audience, however, there are a couple of tricks you can already try.


Start your subject with an action word. You're creating a call to action in a way. Being asked to do something is of course not magical but it does contribute to the impulse to interact with the message:

  • ‘Discover our….’
  • ‘Learn about…’
  • ‘Get certified in…’
  • ‘Sign up for our…’
  • ‘Get the latest…’
Play with clickbait

Clickbait is tricky to work with but it might boost up your results temporarily or even long term if you do it right. Try injecting some shock value or creating an aura of mystery around the subject of your message:

  • ‘You will never believe…’
  • ‘Don’t miss out on…’
  • ‘This is urgent:…’
  • ‘OH NO!: …’

Use personalisation via placeholders to start your subject with the name of your contact:

  • ‘Martha, this is just for you!’
  • ‘Thank you, Tom!’
  • ‘Noah, we need you.’
  • ‘Julia, your results are ready.’
  • ‘Emma, we miss you.
  • 'Best deals in Antwerp right now'

Use the fear of missing out and put a time constraint on your offer:

  • ‘Today only!’
  • ‘You have 24 hours to …’
  • ‘Limited time offer for …‘
  • 'Don't miss out on ...'

Asking a question in your subject line will spark the interest in your contacts to click and open the email:

  • ‘Guess what genius move protects you from scam’
  • ‘Wanna know the number one mistake in choosing a prom dress?’

Include statistics or lists in your subject line. With that, you create the feeling that the email is bringing a valuable piece of information. It will raise the curiosity in your contacts:

  • ‘1 in 5 marketers makes these mistakes’
  • ‘Avoid these 5 most common marketing mistakes’

If you use numbers, make sure type the digits in, and not to spell them out. This is how they will stand out.

Something funny

Funny and punny subject lines will not only raise your open rates but they will also improve the mood of your contact. They will be more receptive to your message when they already wear a smile on their face.

  • ‘It is qualiTEA time’
  • ‘Purr-fect cat food for your furr-ball’
Content hint

Do tell your audience what they will find in the email:

  • ‘Our promotions this week’
  • ‘Download your free whitepaper’
Bonus points: Preview

Use Message Check for a sneak peak at your preview line. Next to your subject line, your contacts will see the first lines from your content even before they open the message. Make your preview complement the subject line. It must follow the same rules.

A few things to avoid (spam markers)

Any of these four details can be your one-way ticket to the spam folder.

Avoid overly promotional lines

Any subject line that is too pushy will put your audience off:

  • ‘Buy now!’
  • ‘Free...!’
Questions followed by exclamations

There is a mail client protocol (PLING_QUERY) that tests emails against this rule. You cannot have a question immediately followed by an exclamation OR vice versa. If you do, you are marked as a spammer and your message will not reach your contacts.

  • ‘Looking for a good time? Check this out!’
  • ‘Want to earn extra? Click here!’

Subjects like these are bound to fail you.


It is clear that you want to stand out. However, using all caps in your subject line is perceived as too forward. If spam filters let you go, the user might mark your message as spam manually.

False promises

Avoid false promises at all cost. Be very careful when you play with clickbait, too. You need to be able to walk the fine line between keeping it interesting to the fullest and raising the bar so high in the subject that you are not able to follow through with your content once your contact opens their message.

A few tricks to improve

What subject works best depends largely on your audience. There are two things you can do to make sure you are working towards improving your results:

Campaign reports

In your campaign reports rates, you will discover the open rate. It will tell you how many of the contacts actually opened the emails you have sent them. It is a very telling metric of the effectiveness of your subject line.

You can also compare the rate of the current campaign to the typical rate for your business. If this rate goes above average, it is worthy to consider setting a new trend for the general appearance of your subject lines based on this successful attempt.

AB campaign

Flexmail offers you automated AB testing based on a subject line. You can create two identical campaigns which only differ on the subject line and a segment of your contacts that will be the test group. Half of your test group receive subject line A and the other half – subject line B. After a period of time, predetermined by you, the rest of your contacts will receive the e-mail with the subject line that triggered most opens or clicks.

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