Test curiosity gap subject lines on the next step
The choice is made after the open
A curiosity gap is not an end in itself. Choose it only if, in a fair comparison, it delivers more of the next step that matters for this campaign, such as a click to an offer, a reply, or an enquiry. A higher open rate is at most an intermediate result. A clear subject line may therefore be the better choice when it aligns more closely with what the recipient needs to do next. This is an editorial decision rule for your own measurement, not a general performance claim: the supplied passages do not compare subject line types.
Create one comparison you can repeat
For one existing campaign, use two variants with the same sender, preview text, email content, audience, and send time. Variant A states the message clearly; variant B leaves one relevant element open. Define the decisive next step in advance and assess both variants against it. Use this test card: campaign, segment, clear variant, curiosity-driven variant, decision step, outcome, owner, and next action. That way, a striking open rate does not replace the action you intended to measure. The source passages do provide operational context: M3AAWG says the documents help bulk mail senders and email service providers implement best practices that help reduce spam and the spread of malware, and describes authentication as a complementary effort to identify trustworthy email participants.
Do not draw a broader conclusion than your test supports
A winner in one mailing is not a universal formula. Repeat the same comparison with a subsequent relevant segment before applying the approach more broadly. Also check whether the promise in the subject line matches the email content. The supplied sources support sender best practices, authentication, and message validation, but not which curiosity gap subject line performs better or how much revenue it generates. DMARC.org mentions services where a message is evaluated according to several authentication systems; that is not a standard for your subject line test, sample size, or commercial outcome.
