Email Subject Line Best Practices for AI-Sorted Inboxes

On October 12, 2026, Google began rolling out Gmail’s AI Inbox to paid Workspace customers, a view that pulls out “action items, deliverables, and deadlines” and summarizes the reader’s most important topics, according to its Workspace update. It is available in US English only. Apple got there first: since October 2024, Apple Mail on Apple Intelligence devices has let readers “see summaries without needing to open a message,” Apple said.

What AI inboxes change

In Gmail’s main list, readers still see sender, subject and your preheader. The summaries Gmail made free in January 2026 appear at the top of long threads after they are opened, and AI Inbox is a separate view that surfaces to-dos and topics, favoring people the reader emails often.

Apple Mail goes further. On Apple Intelligence iPhones, iPads and Macs, the AI summary replaces the preview line, and late-2024 tests by Knak found that a preheader “cannot influence the summary,” which is built from the sender name, subject and body text. On those devices, the sender name and subject line are the only text you still control word for word. So write it to be understood, not to tease, and put the offer or date in the first lines of the email.

Length, preheaders and names

Mobile inboxes show about 25 to 30 characters of a subject line and desktops about 60, according to Litmus. Mailchimp recommends “no more than 9 words and 60 characters.” Put the meaning in the first 30 characters.

Keep writing preheaders. Gmail, most Outlook inboxes and older iPhones still show them, and one that adds a fact the subject leaves out beats a repeat. Don’t pad them with hidden text: Google’s sender guidelines warn that using HTML and CSS to hide content “might cause messages to be marked as spam.”

On names, the evidence is mixed. In field experiments published in Marketing Science in 2018, adding the recipient’s name raised the chance of an open by 20% and sales leads by 31%. A 2023 study in Marketing Letters found “no indication” of a lift in an alumni mailing. Personalization that adds information, such as a city or last order, is safer.

Spam words are mostly a myth

Google’s guidelines list no trigger words. They tie placement to authentication, reputation and spam reports: senders should keep reported spam below 0.10% and never reach 0.30%. What the guidelines do ban is deception, such as starting a subject with “Re:” or “Fwd:” when the message is not a reply. US law agrees: the FTC says a subject line “must accurately reflect the content of the message.”

How to test subject lines when opens lie

Apple’s Mail Privacy Protection preloads emails, so they look opened on delivery. Apple accounted for 76.56% of tracked opens in September 2026, and Litmus says privacy-protected opens “are not considered reliable.” An A/B test judged on opens is skewed by Apple’s machine opens.

Judge winners on clicks or revenue instead. Mailchimp lets you test up to three variations of a subject line and pick the winner by click rate or revenue. If you must test opens, run the test only on readers who don’t use Apple Mail. Test one change at a time.

Subject lines are one part of our email marketing guide. See also Gmail’s subscription manager and our pieces on marketing psychology and outreach emails. We write and test subject lines as part of our email marketing and conversion rate optimization work.

Frequently Asked Questions

How long should an email subject line be?

Short enough that the meaning lands in the first 25 to 30 characters, which is roughly what a phone shows. Litmus says desktop inboxes show about 60 characters, and Mailchimp recommends no more than nine words and 60 characters. A 2014 Retention Science study found six to ten words performed best in retail email, but it predates Apple’s privacy changes, so treat word counts as a guide and test your own list.

Do Apple Mail’s AI summaries replace the preheader?

On Apple Intelligence devices with summaries switched on, yes. The summary takes the place of the preview line, and vendor tests found the preheader cannot steer it. Gmail, most Outlook inboxes and older iPhones still show your preheader as written, so keep writing one. Because the summary draws on the body, put the key offer, date or number in the first lines of the email as well as in the subject.

Are there spam trigger words to avoid?

Not in Google’s rules. Its sender guidelines list no trigger words; Gmail filtering rests on authentication, domain reputation and how many readers report spam, with a ceiling of 0.30%. What Google does ban is deception, such as a fake “Re:” prefix. Under US law, a subject line must reflect the content of the message, and the FTC lists penalties of up to $53,088 per violating email.

Does putting the first name in a subject line still work?

Sometimes. A 2018 field study in Marketing Science found that adding the recipient’s name raised opens by 20% and leads by 31% and cut unsubscribes. A 2023 study found no lift from first names in an alumni mailing. The result depends on the list and the sender, so test it, and favor personalization that tells the reader something, such as their city or their last order.