Here's a humbling truth about email marketing: your carefully written email doesn't exist for a reader until the subject line earns the open. I learned this the way most people do — by sending good emails with forgettable subject lines and watching the open rates explain the problem.
A strong subject line does three jobs: it stands out in a crowded inbox, it creates a reason to open, and — the part beginners skip — it makes a promise the email actually keeps.
Here are the five practices that reliably work, including how to use AI without letting it flatten your voice.
Keep it short and clear
People skim inboxes in seconds, mostly on phones that truncate anything long. Aim for under 50 characters, roughly six to ten words.
Weak: "Discover Our Latest Collection and Enjoy a Limited-Time 20% Discount on All Products!"
Strong: "20% off — today only."
The short version isn't less informative. It's more findable.
Spark curiosity — honestly
Curiosity opens emails: "You're making this email mistake." "What no one told you about pricing." The mechanism is a small gap between what the reader knows and wants to know.
The line to never cross: a great subject line is a promise — make sure your email delivers on it. Curiosity that resolves into value builds trust; curiosity that resolves into a bait-and-switch is one of the classic mistakes that costs you the relationship for one open.
Personalize where it's real
"Sarah, here's the guide you asked about" outperforms generic blasts because it is actually about Sarah. Names help; behavior-based personalization helps more — referencing what the subscriber downloaded, browsed, or bought.
The test: would this subject line make sense sent to only this person? If yes, it's personalization. If it's a name bolted onto a blast, readers can tell.
Use urgency only when it's true
"Last chance: 50% off ends tonight" works when it ends tonight. Time-sensitive language drives action precisely because it carries information — spend that power only on real deadlines, or your urgency stops meaning anything.
Use AI as your brainstorming partner
Here's where generative AI genuinely earns its place in email marketing: subject lines are a volume-and-selection problem, and AI is excellent at volume. Ask for fifteen options for your announcement — specifying the audience, the offer, and the tone — and you'll usually find two or three worth refining.
Two rules keep this working. First, AI brainstorms; you choose. The machine doesn't know your audience's history or your brand's line on hype — your judgment is the filter, and it stays that way. Second, the quality of the options depends entirely on what the AI knows about your business: context determines quality, for subject lines exactly as for everything else. A prompt that includes who your subscribers are and what they signed up for produces options in your world; a bare prompt produces everyone's subject lines.
Starting today
Keep it under 50 characters. Spark honest curiosity. Personalize where it's real. Save urgency for true deadlines. And let AI widen your options while you keep the final call.
Small changes, measured in open rates — and open rates are where everything else your list can do begins.
Frequently asked questions
How long should an email subject line be?
Under 50 characters — roughly six to ten words — so it survives mobile truncation and inbox skimming.
Can AI write good email subject lines?
AI is excellent at generating options, especially when told your audience, offer, and tone. The human job is selection: choosing the line that's true to the email and your voice.
Do urgency subject lines still work?
Yes, when the urgency is real. Genuine deadlines drive action; manufactured urgency trains subscribers to ignore your deadlines entirely.
If your email program needs more than better subject lines — the list, the sequences, the follow-through working as one system — that's infrastructure: explore operating infrastructure.