A subject line is a promise the mail has to keep.

Look at any inbox and you will find the same small tricks. A “Re:” on a mail nobody replied to. “Urgent” on a newsletter. “Quick question” above four paragraphs and a calendar link. Each one is a tiny lie, and each one gets opened, because the reader can't tell it is a lie until they tap.

Then they can. And the next time your name turns up in their inbox, they remember the tap.

The small lies, and what they cost

• The fake reply. “Re: our conversation” when there was no conversation. It borrows a relationship you don't have, and the reader notices in the first line.
• The false alarm. “Urgent”, “Action required”, “Final reminder” on something that is none of those. People who are busy have learnt to read these as spam, which is where they then file you.
• The quick question. If the question is quick, ask it in the subject. If it isn't, the subject was the first untrue thing you told them.
• The fright. A legal notice, a raid, a complaint number. We sent a few of these ourselves, and wrote in the 27 cold mails why we would delete them now. A joke is fine. A dropped stomach is not.

All four buy an open with trust. The open is worth a few seconds. The trust was worth every mail you send that person afterwards.

What an honest subject line does

It tells the reader two things: what is inside, and why it is for them. That sounds dull, and written lazily it is. “Content writing services” is honest and nobody will open it, because it could be addressed to anyone.

The fix is to be specific, and specific is rarely dull. To a dental clinic in Navrangpura: “Your Google listing says you're closed on Saturdays.” To a pickle brand: “The Gujarati on your jar and the English on your site disagree.” Both are true, both can be checked in ten seconds, and both are more interesting than “Urgent”, because they are about the reader.

A subject line can be odd, rude or too casual. What it can't be is untrue, because the reader finds out in one tap.

Clever is allowed, if it pays off

None of this means subject lines have to be plain. Our mail to Beardo, a grooming brand, had the subject “ive gone bald”. It is strange, and it is also true to the mail: the first line explains it. The test for a clever subject is whether the first line inside makes the reader smile or frown. A smile means the subject kept its promise. A frown means it was bait.

The first line matters as much as the subject, because on a phone they arrive together. We wrote about that pairing in the cold mail first line.

The mails that aren't cold

Most of the mail a business sends goes to people who already know it: quotes, invoices, follow-ups, drafts for approval. Here the subject line has a second job. It has to be findable in three weeks, when someone types into the search bar looking for it.

• Name the thing. “Quote: shop fit-out, Patel Opticals, CG Road” finds itself. “As discussed” is lost by Friday.
• Say what you need. “Revised floor plan for CG Road, approve by Thursday” tells them what to do before they open it.
• Make a follow-up carry something new. “Following up” tells them only that you are waiting. “One change to the quote, and a question about dates” gives them a reason to open.

The contact form on this site fills in the subject “Hiring the pen” before anyone types a word. Three plain words, and the mail says what it is before anyone opens it.

Write three, then pick

When we wrote the hospital mails for SELEQT's awareness programme, several went out to the client with three subject lines each, so they could choose how much risk they were comfortable with. That habit is worth stealing for your own mail.

1. A plain one. What is inside, in as few words as possible.
2. A specific one. Something only this reader could receive.
3. An odd one. The one you are slightly nervous to send.

Then pick by the reader. A trust hospital and a sneaker brand should not get the same risk level. Whichever you pick, read it once more and ask whether the mail underneath delivers what the subject said. If it doesn't, change the subject and leave the mail alone.

This week, open your sent folder and read the subjects of your last ten mails, nothing else. Count how many tell you what was inside. The ones that don't are the ones your reader skipped, or will never find again.