Your email subject line determines whether customers open your message. A clear, engaging subject line improves open rates, avoids spam filters, and drives engagement.
This article shows you how to write subject lines that get results and where to add them in Omnisend.
Before You Begin
There's no character limit for subject line length, but inboxes display only a shortened preview. You can check the inbox preview right next to your campaign's subject line field in Omnisend.
Different email clients show different character counts:
Email Client | Character Length Displayed |
Outlook 2010 preview | 54 characters |
Outlook 2010 compact | 73 characters |
Thunderbird | 66 characters |
Gmail | 70 characters |
Outlook.com | 60 characters |
Yahoo Mail | 46 characters |
Android (480 x 320px) portrait | 27 characters |
Android (480 x 320px) landscape | 46 characters |
Android (800 x 480px) portrait | 30 characters |
Android (800 x 480px) landscape | 62 characters |
iPhone portrait | 41 characters |
iPhone landscape | 64 characters |
Windows Phone portrait | 42 characters |
Windows Phone landscape | 61 characters |
Why Subject Lines Matter
The subject line is the first thing subscribers see in their inbox. It influences:
Open rates – A strong subject line increases clicks.
Deliverability – Spammy phrasing sends emails to junk folders.
Brand perception – Consistency builds trust.
Focus on clarity, relevance, and urgency to improve performance.
Subject Lines in Campaigns and Automations
When you create an Email Campaign, the subject line is the first field in Email settings. Fill it out before designing your message.
In an Automation, click the email block in your workflow. Edit the Subject Line in the right sidebar.
💡 Tip: Use Omnisend AI to generate subject line ideas instantly. Click the AI icon next to the subject line field, describe your campaign, and get 3 customizable suggestions.
Subject Line Best Practices
Keep It Short and Simple
Email clients truncate long subject lines. Write your key message in the first 40–50 characters so it's visible on mobile devices.
Use simple, scannable language. Avoid long or complex phrasing.
Examples:
"You're invited!"
"Holiday Special"
"Are you coming?"
"Still thinking about it?"
"Here's Why We [Something Remarkable]"
"Last call: [Name Of Product] closes in [#] hours"
"Your discount code inside"
"Last call: Sale ends tonight"
"New arrivals just for you"
Use Personalization
Personalized subject lines increase open rates. Add the recipient's name, location, or purchase history using Omnisend's personalization tags.
How to Add Personalization:
Click the tag icon in the subject line field.
Select a dynamic field (e.g., First Name, Location).
Examples:
“[Name], meet [your new product/feature]”
"We'd love your feedback, [Name]"
"[Name], we picked these for you"
"Only the best for you, [Name]"
"Be stylish in [Location]"
"Proof That [Product] Works"
"[Name], this is so you, and it's 70% off!"
"Try Out [Product] For Yourself"
💬 Learn more: Personalization in Email Messages.
Add Numbers & Lists
Numbers make subject lines scannable and intriguing. Use numerals (not spelled-out numbers) to save space.
Examples:
"25% off your favorites"
"Two for two"
"Top 10 products in [Company]"
"9 Facts about [Product]"
"25% off your favorites"
"Back By Demand 25% Off"
"Spend 60 Minutes or Less with [Product]"
"21 ideas how to use [Product]"
Use Punctuation Strategically
Question marks and exclamation points add emphasis—but use no more than 2–3 punctuation marks per subject line. Too many trigger spam filters.
Examples:
"[Product] Restocked and Price Drop!"
"New colors in old favorites!"
"Are you ready for the sale?"
"Shhh! Don't tell anyone about this discount!"
"You don't want FOMO, do you?"
"Hey, forgot something? Here's 20% off."
"Quick favor?"
"[Name], do you remember us?"
⚠️ Warning: Special characters (* % &) can trigger spam filters. Use sparingly.
Create Urgency with CTAs
Tell recipients what to do next. Add time-sensitive language to drive action.
Examples:
"Last call: [Name Of Product] closes in [#] hours"
"Talk to Us"
"Give [Product] a Try"
"Be Awesome with [Product]"
"Save 20% with [Company]"
"Act fast and save 20% on [Product]"
"Hurry! Your discount is about to expire"
"Don't miss this deal"
"See what's on sale instantly"
"Claim your coupon now!"
"Reveal your offer today"
"Start saving on [Product type] today"
Avoid Spam Triggers
Certain words and formatting send emails to junk folders. Avoid:
ALL CAPS – Use capitals only for a single word (e.g., "SALE ends tonight").
Excessive punctuation – No more than 2–3 punctuation marks per subject line (e.g., "!!!!!").
Spam trigger words – "$$$," "100% free," "cash off," "cheap."
Misleading content – Subject line must match email body.
"Re:" or "Fwd:" – Don't fake replies or forwards.
Overly long lines – Keep your subject lines under 70 characters for best visibility.
⚠️ Impact: Spammy subject lines damage your sender reputation and reduce deliverability.
💬 Learn more: Optimize Email Content for Improved Deliverability.
Tools to Test & Optimize Subject Lines
Omnisend AI Subject Line Generator
Omnisend AI generates subject lines based on your campaign topic.
How to Use:
Create an Email Campaign or Automation and click the AI icon next to the subject line field.
Enter context about your campaign (e.g., "Black Friday sale, 30% off").
Omnisend AI suggests 3 subject line variations.
Refine using default prompts (change tone, translate, shorten, rephrase) or add custom instructions.
You can also generate AI-powered preheaders that complement your subject line.
💬 Full guide: Omnisend AI: Email Optimization.
Subject Line Tester
Use Omnisend's Subject Line Tester to check length, wording, and spam score. It shows an inbox preview and suggests improvements.
A/B Test Subject Lines
Test subject line variations to see what resonates with your audience.
How to Set Up A/B Testing:
Go to Campaigns → New Campaign → Email A/B Test.
Create 2+ subject line variants.
Omnisend sends each to a segment and tracks opens.
💡 You can also A/B Test subject lines in Automation. Set Up A/B Test Splits in Automations.
Additional Resources
If you want to learn more about subject lines and different strategies, we suggest checking our blog posts:
FAQ
What's the ideal subject line length?
5–7 words or 40–70 characters, depending on email client. Write key info first so it's visible on mobile. There's no hard character limit, but inboxes truncate previews.
Should I use emojis in subject lines?
Yes, if relevant to your brand. Use 1–2 emojis maximum and test across email clients to ensure they display correctly. Learn How to Use Emojs in Emails.
How do I check my subject line performance?
Use Omnisend's Campaign Reports to track open rates. A/B test different subject lines to identify what works best for your audience.
What words should I avoid to prevent spam filtering?
Avoid ALL CAPS, excessive punctuation (!!!), and overused sales words like "free," "buy now," "limited time," "cash," or "prize." Don't use misleading subject lines that don't match your email content. Use the Subject Line Tester to check for spam triggers.
What is preheader text, and how is it different from the subject line?
Preheader (preview text) is the short line that appears after your subject line in the inbox. Add it in the email Settings field (not in the email builder). Keep it 30–80 characters and make it complement your subject line.
Example: Subject: "50% off today" → Preheader: "Sale ends at midnight!"
Can Omnisend AI write subject lines for me?
Yes. Click the AI icon in the subject line field, describe your campaign, and Omnisend AI will generate 3 suggestions you can customize.
Any questions? Email us at [email protected] or chat with us in-app.