Email Subject Line Best Practices: How to Write Subject Lines That Get Opened

Your subject line is a gatekeeper. It determines whether someone opens your email or ignores it. With the average person receiving 121 emails per day, yours needs to stand out—without triggering spam filters.

Why Subject Lines Matter for Deliverability

Subject lines affect deliverability in two ways:

• Direct: Spam filters scan subject lines for trigger words
• Indirect: Low open rates signal to ISPs that your emails aren't wanted

Gmail, Outlook, and Yahoo track engagement. If nobody opens your emails, they assume recipients don't want them and start filtering to spam.

Subject Line Best Practices

1. Keep It Short

Aim for 30-50 characters. Mobile devices truncate longer subject lines.

Good: "Your Q4 analytics are ready"

Too long: "Your comprehensive quarterly analytics report for Q4 2024 is now available for download"

2. Front-Load Important Words

Put the most important information first, in case it gets cut off.

Good: "Meeting tomorrow: Project X kickoff"

Bad: "Just wanted to remind you about our meeting tomorrow regarding Project X"

3. Personalize When Possible

Including the recipient's name or company can boost open rates by 26%.

Examples:

• "Sarah, your trial ends Friday"
• "Acme Corp: Your invoice is ready"
• "Quick question about [Company] website"

4. Create Curiosity

Make them curious enough to open, without being clickbait.

Good:

• "The one metric that matters"
• "You're doing this wrong"
• "3 things we changed that doubled conversions"

Clickbait (avoid):

• "You won't believe what happened next"
• "This one weird trick"
• "Shocking truth revealed"

5. Use Numbers and Lists

Numbers catch the eye and set clear expectations.

Examples:

• "5 ways to improve deliverability"
• "3-step checklist for cold emails"
• "47% of emails fail this test"

6. Ask Questions

Questions engage and make people think.

Examples:

• "Are your emails reaching the inbox?"
• "Ready to 2x your open rates?"
• "Still using [old method]?"

Subject Lines to Avoid

Don't Use ALL CAPS

ALL CAPS looks aggressive and spammy. Even one word in caps can trigger filters.

Don't Overuse Punctuation!!!

Multiple exclamation marks scream spam. One is plenty.

Don't Use Too Many Emojis

One emoji can work. Multiple emojis look unprofessional.

Don't Be Misleading

Your subject line must match your content. "Re: Your order" when there is no order is illegal under CAN-SPAM.

Subject Line Formulas That Work

The Question

"[Question about their pain point]?"

Example: "Tired of emails landing in spam?"

The Number

"[Number] [Thing] to [Desired outcome]"

Example: "7 words that trigger spam filters"

The How-To

"How to [achieve goal] in [timeframe]"

Example: "How to improve deliverability in 5 minutes"

The Personal

"[Name], [specific reference]"

Example: "John, saw your post about email marketing"

A/B Testing Subject Lines

Always test subject lines before sending to your full list. Test variables one at a time:

• Length (short vs. long)
• Personalization (name vs. no name)
• Emoji (with vs. without)
• Question vs. statement
• Numbers vs. no numbers