When should you use Subject line fits the inbox?
email-subject-line-length · functional · recommended
When should you use this check?
Run this on every campaign where the subject is doing sales work, which is most marketing and all cold outreach. It is most useful when subjects carry personalisation, because a token that reads as fifteen characters in the editor becomes thirty for some recipients and pushes the point of the line out of view. Include it whenever subject variants are being tested, since variants are written quickly and length is the thing nobody re-measures.
When should you not?
Skip it for transactional mail whose subject is a functional label — an order number, a reset notice — where being cut off costs nothing because the recipient already knows what the message is. It is also the wrong check if your question is whether the subject is *good*; length is a mechanical constraint, not a judgement about the copy. And if you are targeting a specific client's exact rendering, note that this measures characters rather than pixels, so a wide-character subject can still be cut early.
What does it inspect?
Extract Subject header; count Unicode characters; fail when count exceeds maxSubjectLength.
What does a failure mean?
A failure means the subject exceeds the length that reliably survives the inbox, and it reports the measured length. Inboxes cut long subjects at different points — desktop Gmail shows roughly seventy characters, mobile apps as few as thirty to forty — so a long subject is not invisible, it is unpredictable. The practical consequence is that whatever you put at the end of the line is read by some recipients and not others, and the ones on mobile are usually the majority.
What are the Standard defaults — and why?
One parameter carries the check: `maxSubjectLength`, defaulting to `60` characters. Sixty sits deliberately below desktop Gmail's roughly seventy, because the boundary that matters is the mobile one and most opens happen there — a subject that fits sixty survives nearly everywhere, while one tuned to seventy is cut on phones. It is a single one-sided limit rather than a range, since there is no failure mode in a subject being short.
How does an agent call it?
{
"type": "email",
"validations": [
"email-subject-line-length"
]
}
Who governs the Standard?
Schemafirst.org publishes community-governed standards for digital QA. ArbiterQA is a sponsor and commercial licensee of those standards; citation does not mean Schemafirst operates ArbiterQA.
Author ArbiterQA · Reviewed by ArbiterQA · 2026-08-14