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Email Marketing Glossary.

Clear, no-fluff definitions of every email marketing term that matters—from open rates to double opt-in. 

 

Subject Line

A subject line is the short line of text a recipient sees before opening an email. It's the single biggest factor in whether your email gets opened or ignored.

Also known as: email subject line, subject

Your subject line is the gatekeeper of the inbox. No matter how good the email is, it won't perform if no one opens it — and the subject line is what earns that click.

How long should a subject line be?

Aim for roughly 40–50 characters (about 9–14 words) so it doesn't get cut off on mobile. Front-load the most important words in case it's truncated.

Subject line best practices

Be specific and honest, use numbers where relevant, personalize beyond just a first name, and A/B test regularly — there's no universal formula. A single emoji, placed at the start or end, can lift opens when it fits your brand.

Before you hit send, run your line through Drip's free Email Subject Line Tester for an instant score and specific fixes.

How do you avoid the spam folder?

Skip aggressive sales words ("buy now," "act now"), ALL CAPS, and excessive punctuation. Most importantly, make sure the subject matches the email's content — mismatched promises drive complaints that damage your sender reputation.

Related terms

Call to Action (CTA) Preheader Email Template Personalization Dynamic Content

Related reading from Drip

9 Email Subject Line Best Practices For High Open Rates 180+ Proven Email Subject Lines (Sorted by Category)

Frequently asked questions

What is an email subject line?
It's the short line of text shown in the inbox before an email is opened. Along with the sender name and preheader, it's what a recipient uses to decide whether to open, ignore, or delete your email.
How long should an email subject line be?
Around 40–50 characters, or about 9–14 words, keeps it from being cut off on mobile devices. Put your most important words first in case the line is truncated.
How do you write a good subject line?
Be specific and honest, hint at clear value, use numbers or personalization where relevant, and A/B test versions. Make sure it accurately reflects the email's content so you don't trigger spam complaints.
What subject line words trigger spam filters?
Aggressive phrases like "buy now," "act now," and "click here," plus ALL-CAPS words such as FREE, CASH, and URGENT, and heavy punctuation can flag filters. Clear, honest wording and an authenticated domain keep you out of spam.
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