Free tool Email subject line tester with inbox previews
Before you send a campaign, a product update or a newsletter, test the subject line and preview text: where each inbox cuts them off on phone and desktop, which public spam-filter rules they match, and the classic slips such as a fake "Re:", all caps or an unreplaced merge tag. Every check names its source, and nothing is scored.
- Previews for 4 email clients
- Every rule cites a source
- Runs locally
Test a subject line
Enter the subject, preview text and sender name. Results update as you type.
Paste it from your email platform. Merge tags such as *|FNAME|* or {{name}} can stay; the tool will flag them.
The snippet shown after the subject in the inbox. Leave it empty to see what happens without one.
The name recipients see in their inbox.
How it looks in the inbox
Gmail on the web
Gmail app (phone)
Apple Mail (iPhone)
Outlook (desktop)
Widths are estimates from common layouts: desktop assumes a browser window about 1280 px wide, phones a screen about 390 px wide, with Arial glyph widths. The real result depends on the client, font, screen and settings, so send yourself a test email and check it on a phone.
Subject length
46
Preview text length
0
Emoji in subject
0
Checks
- CheckGets cut off in Gmail app (phone), which shows only about the first 35 characters. Put the key information first.
- NoteNo preview text. Most clients fill that space with the first text in the email body, often "View this email in your browser".
How much each client shows
"First ~N chars" is estimated from display width.
| Client | Subject | Preview text |
|---|---|---|
| Gmail on the web | Full | Shows body text |
| Gmail app (phone) | First ~35 chars | Shows body text |
| Apple Mail (iPhone) | Full | Shows body text |
| Outlook (desktop) | Full | Shows body text |
Version v1.0Updated Update history
Three things decide whether people open
Sender, subject and preview text. The tool checks how each displays in the inbox and whether it uses wording that public rules discourage.
Inbox previews
Four common list layouts: Gmail on the web squeezes subject and preview onto one line; phones give the subject its own line and the preview one or two. Check that the words that matter stay visible.
Cut-off point
The table shows roughly how many characters each client displays. It is estimated from display width, so phones and font-size settings will vary.
Spam signals
Every flagged rule has a source: public SpamAssassin rules or Gmail's sender guidelines. "Common promotional phrasing" is only a note; it will not send mail to spam by itself.
Preview text and sender
Without preview text, clients fill the space with the start of the body; repeating the subject wastes it. Gmail advises against putting promotional text in the sender name.
Inbox placement depends far more on domain authentication (SPF, DKIM, DMARC), consent and complaint rates than on the subject line. TapCub does not measure email opens; it does show what people do after they click through to your site.
Three steps before you hit send
For campaigns, lifecycle emails and newsletters.
-
1
Enter subject and preview text
Copy them from your email platform. If you have not set preview text, leave it blank to see what shows instead.
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2
Tick the China box if it applies
For commercial email to recipients in mainland China, tick the box to check for the required 广告 or AD prefix.
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3
Read the cut-offs and checks
Make sure the key words survive on a phone, deal with any amber rules, then send a test to a real device.
Rules and sources behind every check
Only rules with a findable source are stated as rules; anything else is marked "widely seen as" with the reason (checked 2026-11-10).
- Display width: subject and preview text are measured with Arial glyph widths against the client widths in the table below. TapCub estimated these widths from common layouts; clients do not publish them
- SpamAssassin: the subject regexes from its public rules file (
20_head_tests.cf) are used as-is;SUBJ_ALL_CAPSfollowsHeaderEval.pm: after removing Re: prefixes, more than one word, at least 10 letters, all uppercase - Gmail sender guidelines: do not start subjects with Re: or Fwd: unless they are real replies or forwards; do not use emoji to imitate graphic elements; keep subject and content out of display names; headers and content must not be misleading
- US law: the FTC's CAN-SPAM compliance guide says the subject line must accurately reflect the content of the message. China: Article 13 of the Internet Email Service Measures requires explicit consent and a 广告 or AD prefix for commercial email
| Client | Subject | Preview text |
|---|---|---|
| Gmail on the web | 14 px, sharing one line of about 640 px with the preview | Follows the subject in the remaining width |
| Gmail app (phone) | 16 px, one line of about 280 px | 14 px, one line |
| Apple Mail (iPhone) | 15 px, one line of about 330 px | 15 px, two lines |
| Outlook (desktop) | 14 px, one line of about 300 px | 13 px, one line |
What the sample subject turns up
The default is a last-day sale email: a 46-character subject with no preview text.
| Check | Result | Fix |
|---|---|---|
| Phone display | The Gmail app shows about 35 characters, so "ends tonight" is lost | Lead with the deadline: "Ends tonight: 50% off annual plans" |
| Preview text | Empty, so the body text shows instead | Add what the subject leaves out: "Lock in this year's price for both analytics and live chat" |
| Spam-filter rules | No SpamAssassin subject rules matched | — |
| Sender name | Short and clear | — |
After the rewrite the deadline is visible on every phone, and the preview line carries the detail instead of repeating the subject.
Four things that slip through before sending
Most of them are invisible in a desktop preview.
Only checking on desktop
Webmail shows long subjects; phones often show half. Always look at the test email on a phone.
Leaving the preview text empty
Without it, the inbox may show "View this email in your browser" or a run of links.
Faking "Re:" or urgency
One extra open costs unsubscribes and complaints. Gmail explicitly says not to fake replies, and complaints hurt every email that follows.
Merge tags without a fallback
"Hi *|FNAME|*" goes out as-is, or becomes "Hi ,". Set a default value before sending.
After the click, TapCub takes over
The subject line decides the open, and open rates live in your email platform. What people do once they click through to your site shows up in TapCub.
Campaigns and UTM
Tag email links with utm_medium=email and a campaign name, then split visits and conversions by campaign, source and content to see which button or version worked.
Learn moreFirst and last touch
Email is often the first touch while the purchase comes later through search. First-touch and last-touch views side by side keep email from being undervalued.
Learn moreLanding page report
Bounce and conversion on the pages email sends people to, compared with other channels.
Learn moreSubject line tester FAQ
Still have a question?
Chat with the team behind TapCub — we usually reply within a few hours on business days.
How long should an email subject line be?
There is no single number; it depends on the client. In common layouts phones show roughly 30–40 characters of the subject, while webmail shows much more. This tester estimates the cut-off for each client from display width, which is closer to reality than counting characters.
Which words send email to spam?
No mailbox provider publishes a spam-word list. Mailchimp notes that a campaign usually needs multiple triggers to be filtered; domain reputation and authentication (SPF, DKIM, DMARC), consent and complaint rates matter far more. The open-source SpamAssassin filter has a few public subject rules, such as all caps or a question mark with an exclamation mark, and the tester flags each one.
What is preview text (preheader)?
The short snippet shown after or below the subject in the inbox list. If you do not set it, clients use the first text in the email. Most platforms have a preview text field; in hand-coded HTML, a hidden line at the top of the body sets it.
Should I use emoji in subject lines?
One or two is fine. Clients render emoji differently and some older ones show a box. Gmail's sender guidelines warn against using emoji or other non-standard characters to imitate graphic elements.
Is "Re:" in a subject line allowed?
Only on real replies and forwards. Gmail's sender guidelines say so explicitly, and the FTC's CAN-SPAM guide requires subject lines that accurately reflect the content. A fake reply may earn an open, then a complaint.
Do I need to mark ads when emailing people in China?
Yes, for commercial email. Article 13 of China's Internet Email Service Measures requires explicit consent and 广告 ("advertisement") or AD at the start of the subject. Tick the China option and the tester checks the prefix.
Can this predict my open rate?
No. Open rates need real sends, and Apple Mail Privacy Protection preloads messages, so opens are no longer precise anyway. To compare subjects, use your platform's A/B test and judge by clicks and conversions.
Is my subject line uploaded?
No. Everything runs in your browser. Nothing is stored, added to the URL or sent to TapCub.
Update history
New features, fixes and data refreshes for this tool, newest first.
-
· v1.0
LaunchedFirst release.
Data this tool relies on
- Mailbox provider sender guidelines — checked 2026-11-10, reviewed every 12 months (next review by 2027-11-10)
Missing something or found an error? Tell us — requests that come up often go into the next update. All tool updates
Tag the links in your email
With the subject tested, add campaign parameters to every link so you can see results by campaign once people reach your site.