Email subject line tester: inbox preview and filter checks
This email subject line tester shows roughly how your subject appears in Apple Mail on an iPhone, the Gmail app and Gmail on a desktop, including where it gets cut off. It also checks the line against SpamAssassin's published subject rules and Google's sender guidelines, such as a fake "Re:", and labels common trigger words separately. There's no made-up score.
Results update as you type. Nothing leaves your browser.
Sample result
This subject line starts with a fake “RE:”, which filters and readers distrust.
- Subject
- 41 characters
- Words
- 6
- Emoji
- 0
What this means for you
Drop the RE:, cut the !!, and make sure {{company}} fills in before you send.
- Flags fake Re: and Fwd: prefixes
- SpamAssassin subject rules and caps checks
- Inbox previews for phone and desktop
- Characters, words and emoji counted
- Subject line
- Preview
- Filters
By SimpleVerifier Team · Last updated · Runs in your browser; nothing you paste is sent to us
How do I test an email subject line?
- Type or paste the subject line. Add the preheader and sender name if you have them, because they share the same inbox row.
- Read the three previews. If the line is cut off, check that the words people need come before the cut.
- Read the checks. SpamAssassin rules and Google guideline issues are worth fixing; common-practice flags are judgement calls.
- Check the body copy with the spam words checker before you send.
Example
- You enter
- The subject line “RE: Act now on {{company}}'s onboarding!!”.
- You get
- 41 characters and 6 words with 4 findings: a fake reply prefix (a Google guideline issue), 2 exclamation marks, and the
{{company}}merge tag to check fills in. - Limits
- No limit; runs in your browser.
How long should an email subject line be?
There's no published character limit for inbox display; it depends on the app, the screen and the reader's text size. That's why this tester measures your actual line with the fonts on your device at the width each layout usually gives the subject, instead of quoting a fixed count. A phone shows far less than a desktop, and Apple Mail's default list gives the subject one line, while the preview text below it gets two.
The practical rule: put the words that make someone open it first. If "Sam, 3 ideas for the Q4 launch" is cut after "Sam, 3 ideas", it still works. If "Following up on our conversation last week about pricing" is cut after "Following up on our", it doesn't. The preheader, the text that follows the subject, is the second chance; if you don't set one, the inbox shows the first line of the email.
Will my subject line trigger spam filters?
Only a few subject patterns carry published weight, and none decides delivery on its own. SpamAssassin is the only widely used filter that publishes its rules and scores. Its subject rules include SUBJ_ALL_CAPS (1.506 points with learning and network tests on), a subject starting with a dollar amount (1.8), "Your family" (2.999) and an "ADV" tag (2.899), from its 3.3 rule list. The default spam threshold is 5, so they're shown with their points so you can judge them.
Google's Email sender guidelines say subjects shouldn't be misleading, and specifically not to start one with Re: or Fwd: unless the message is a reply or a forward. For US recipients, the FTC's CAN-SPAM compliance guide also says the subject line must reflect the content of the message. Fake replies are the one subject trick that's both a deliverability risk and a legal one.
Common-practice flags, such as "free" or several exclamation marks, come from cold email advice rather than any published filter rule. Our spam words checker explains the difference in more detail.
What is subject line testing?
Subject line testing means sending two or more versions of a subject to parts of your list and comparing how each performs. The usual measure is open rate, but Apple's Mail Privacy Protection preloads remote content, including tracking pixels, for users who turn it on, which records opens that never happened. Compare subject lines on replies or clicks instead, and send each version to a large enough group that one or two replies don't decide it. Our guide to cold email subject lines has examples.
And test with addresses that work. A subject line that never reached an inbox didn't lose the test, it never took it. Bounces from a stale list also skew the comparison, because whichever version went to more dead addresses looks worse.
Why measure width instead of counting characters?
| Counting characters | Measuring width (this tool) | |
|---|---|---|
| Wide letters (W, M) vs narrow (i, l) | Counted the same | Measured as they render |
| Emoji | One or two characters, depending on the counter | Measured at their real width |
| Different apps | One number for all | A separate result per layout |
| Preheader | Usually ignored | Shown after the subject, as the app does |
Who uses this tool, and for what?
- Cold emailers
- Check that the first words of each subject survive the cut on a phone, and that no step in a sequence uses a fake Re: to look like a reply.
- Marketers
- Preview the subject and preheader together before a campaign, so the preheader adds something instead of repeating the subject or showing "View in browser".
- Agencies
- Paste a client's proposed subjects and show them, app by app, which ones get cut before the point lands.
- SaaS teams
- Test onboarding and announcement subjects for length, then check the body with the spam words checker.
What can’t this tool tell you?
- Previews are estimates. They use typical list widths and your device's fonts, so a reader with larger text or a different app sees a different cut.
- Outlook and other mail apps aren't previewed; only Apple Mail on iPhone, the Gmail app and Gmail on desktop.
- It can't predict opens or replies. A line that passes every check can still be one nobody wants to open.
- The filter checks use SpamAssassin 3.3 subject rules. Gmail and Outlook publish no subject rules to check against.
Common questions
- How many characters show in an email subject line on mobile?
- It depends on the app, the phone and the text size, so there's no fixed number. On a typical phone the subject gets one line of about 280 to 340 pixels. The preview above measures your line at those widths with your device's fonts and tells you roughly how many characters fit.
- What words should you avoid in an email subject line?
- Avoid a fake Re: or Fwd:, all capitals, and the few patterns SpamAssassin scores, such as a leading dollar amount or an "ADV" tag. Beyond those, words like "free" carry no published penalty; drop them only if they make the line read like an ad.
- How do I A/B test an email subject line?
- Send each version to a random, equal-sized part of the same list at the same time, then compare replies or clicks rather than opens. Most email platforms and outreach tools have a built-in A/B option that does the split for you.
- Do emoji in subject lines hurt deliverability?
- No published filter rule penalises emoji in subject lines. They take up width, can render differently across apps, and read as promotional in cold email. The tester counts them so you can decide.
- Is it OK to start a subject line with Re:?
- Only if the email really is a reply. Google's sender guidelines say not to use Re: or Fwd: otherwise, and the FTC's CAN-SPAM guidance says a subject line must not be deceptive about the message's content.
- Is there a subject line score?
- Not here. A score from the subject alone would be invented, because filters weigh sender reputation, authentication and recipient behaviour far more. The tester shows each published rule that matched and its own points, so you can see exactly what it found.
- What is a preheader?
- The preview text shown after or under the subject in the inbox. You set it with a hidden line at the top of the email or in your sending tool. If you don't, the inbox shows the first words of the email instead, often "View in browser" or a greeting.
Sources
Authentication and capacity are half of deliverability.
The other half is who you send to. Every hard bounce counts against the domain and inboxes you are sending from, and nothing in DNS can tell you which addresses on your list are dead.
- Bulk verification with real SMTP mailbox checks
- Hard bounces removed before a send, not discovered after
- Catch-all, disposable and role detection on every row
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