Subject Line
The subject line is the text displayed in a recipient's inbox that summarises the email content and determines whether they open it.
What Is Subject Line?
Subject lines are the single most influential factor in whether someone opens your email. They appear in the inbox alongside the sender name and preview text, competing for attention with dozens of other emails. Effective subject lines are typically 30-50 characters (to avoid truncation on mobile), create curiosity or promise specific value, and match the actual content of the email. Subject line best practices vary by audience — what works for a tech newsletter may not work for a lifestyle brand. The most reliable approach is consistent A/B testing to learn what resonates with your specific subscribers.
Why It Matters for Newsletters
Your subject line is the gateway to all other engagement metrics. If nobody opens the email, no amount of brilliant content matters. Studies show that 47% of email recipients decide to open based solely on the subject line, and 69% report emails as spam based on the subject line alone.
Best Practices
- Keep subject lines under 50 characters to avoid truncation on mobile
- Use specific numbers and data points when possible ('5 trends' beats 'some trends')
- Create urgency without being spammy — deadline-driven is fine, 'ACT NOW!!!' is not
- Personalise with the subscriber's name or preferences when appropriate
- A/B test continuously and track which patterns consistently win
How Aldus Handles This
Aldus's AI generates multiple subject line options for every issue and automatically A/B tests them. Over time, the AI learns which subject line styles perform best for your audience and adjusts its approach — getting smarter with every send.