Subjects that are too short, too long, ALL CAPS, or stuffed with spammy tokens underperform and risk filters.
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Email Subject Line Tester
Campaign workspace
Email Subject Line Tester
Structured inputs and readable results — copy URLs, scan limits, and review findings without digging through JSON.
Recommended ~20–60 characters for many inboxes.
27 / 60
Optional inbox preview text
Results
WaitingResults appear here
Fill the form and run the tool — outputs are structured cards, not a raw JSON dump.
Free · API · MCPToolYour platform
Use Email Subject Line Tester three ways
The free Email Subject Line Tester on this page works in your browser. The same workflow is available via REST API for apps and via MCP for Cursor, Claude, and other agents — one API key, one plan, one quota. This tool is part of the marketing-apis module under Marketing Tools.
- Free in browserUse Email Subject Line Tester here — no account required.
- REST APICall from your server with
X-Api-Key. - MCP for agentsConnect Cursor or Claude to ToolYour MCP with the same key.
- 1. Get an API keySign in and open Dashboard → API Keys (works for REST and MCP).
- 2. Call or connectUse the REST API from your server, or paste the MCP URL into your agent config.
- 3. Same qualityOutput matches the free web tool across all three surfaces.
What is Email Subject Line Tester?
Subjects that are too short, too long, ALL CAPS, or stuffed with spammy tokens underperform and risk filters. ToolYour Email Subject Line Tester scores a subject against length heuristics (recommended ~20–60 chars), flags ALL CAPS, repeated punctuation, money/free cues, and lexicon spam hits. It is not an ESP inbox placement test or A/B predictor. Same route via REST and MCP (POST /api/v1/marketing-apis/email-subject-line-tester).
What are common questions about Email Subject Line Tester?
What does the score mean?
A heuristic 0–100 from length and finding penalties — useful for QA triage, not a guarantee of open rate or deliverability.
Does it check the preheader?
This tool focuses on the subject string. Check preheader separately in your ESP preview; pair with Email Spam Word Checker on body copy.
Can I test multiple subject line candidates at once?
Test one subject per call. For several candidates from an A/B test, run each through separately and compare scores side by side.
What are the key features of Email Subject Line Tester?
Length band
Warns under ~20 or over ~60 characters for common inbox truncation.
Spam-token hints
Surfaces common trigger phrases for rewrite before send.
How do you use Email Subject Line Tester?
Paste the candidate subject
Use the final send string, not an internal draft code.
Address findings
Shorten, drop ALL CAPS, and remove spam hits; re-test.
Why this tool exists
Subject lines that are too long get cut off mid-word on mobile; too short and they look unfinished; ALL CAPS and repeated punctuation read as spammy before a recipient even opens the message. Email Subject Line Tester exists to flag these length and style issues before send, when they're still cheap to fix.
Problems it solves
Marketers often eyeball subject length in a compose window that doesn't show real inbox truncation, and classic spam-trigger patterns (excess punctuation, all-caps words, money/urgency cues) are easy to miss when you wrote the line yourself. A quick heuristic score turns that gut-check into something consistent across a team.
Common use cases
- Agency send QA: scoring a client's draft subject before it goes into the ESP, as a standard pre-send checklist item.
- In-house marketer: comparing two subject line candidates for an A/B test before committing traffic to either.
- Pre-launch: catching an accidental ALL CAPS subject or a length that will truncate on mobile before a newsletter ships.
- Agent/MCP: an agent screening template-generated subject variants and keeping only the ones that pass length and spam-token checks.
Honest scope
The 0–100 score is a heuristic from length and finding penalties — it is not an ESP inbox placement test, does not predict open rate, and does not check your preheader text (verify that separately in your ESP preview). Pair with Email Spam Word Checker for body copy.
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