What is the “Subject” line?

The Subject line is the headline that shows up in the inbox before your email is even opened. It's the first thing (sometimes the only thing) someone sees when deciding whether to read or ignore your message.

Think of it like a newspaper headline. If it's boring or confusing, people skip it. If it's clear and relevant, they click. That's why subject lines matter so much for open rates.

Every email has a Subject line. It's set by the sender when composing the message. Marketing platforms like Mailchimp and Brevo let you test different subject lines (A/B testing) to see which one gets more opens.

Here's what actually shows up in the inbox: the Subject line plus the first bit of preview text (also called the preheader). Most inboxes display both together, so they work as a team. Example:

• Subject: Your order shipped 📦
• Preview: Track your package here. Expected delivery Thursday.

Modern subject lines can include emojis, accented characters, and other non-English text. That works because email clients use MIME encoding (RFC 2047) to translate those characters into a format that works across all systems. When you write "Café☕" in a subject line, it gets encoded behind the scenes so Gmail, Outlook, and Apple Mail all render it correctly.

A few practical tips: keep subject lines under 60 characters (longer ones get cut off on mobile). Don't write subjects in all caps or stuff them with spam trigger words like "FREE!!!" or "Act now!". Those don't land in spam automatically, but they do make readers less likely to open. And if you're stuck on what to write, test your subject line before sending to catch common mistakes.

One last thing: if you send an email with a blank subject line, most inboxes will show "(no subject)" in its place. That's not technically broken, but it looks unprofessional and tanks your open rate.