Email Subject Line Tester
Type a subject line and see it the way your recipients will: truncation on mobile and desktop, spam trigger phrases, shouty capitalization, and punctuation patterns that hurt opens. Analysis runs live in your browser.
0 characters/mobile shows ~35, desktop ~60
What this checks
Length and truncation
Where mobile and desktop clients will cut the subject off.
Spam signals
Trigger phrases, all caps, repeated punctuation, and money framing.
Readability
Emoji density and length extremes that lower open rates.
The subject earns the open. The sender earns the inbox.
A good subject line cannot rescue mail from a bad sending reputation, but a bad one can waste a good reputation. Fix both sides.
Front-load meaning
The first 35 characters must carry the message. Everything after may never be seen.
Write like a person
Sentence case, no exclamation stacks, no ALL CAPS. Filters and readers judge alike.
Match the body
Subjects that overpromise drive spam complaints, which hurt every future send.
Subject line questions.
How long should an email subject line be?
Mobile clients show roughly the first 35 characters and desktop clients around 60. Put the key message in the first 35 characters and keep the whole subject under 60 where you can.
Do spam trigger words still matter?
Modern filters rely mostly on sender reputation and engagement, but classic spam phrasing still correlates with filtering, especially combined with all caps, heavy punctuation, or a new sending domain.
Should I use emoji in subject lines?
One well-placed emoji can lift attention in some audiences. More than that renders inconsistently across clients and starts to look like promotional bulk mail.
See opens, not guesses
SMTPfast tracks opens and clicks per email, so you can compare subject lines on real data instead of rules of thumb.
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