Check Potential Problem in User Content

Before you can send a broadcast in Resend, the interface runs a simple, brilliant check: it scans your email's HTML for an unsubscribe link. If it can't find one, a subtle but unmissable warning appears, preventing you from sending until you've added it.

It’s a safety net that catches you before you fall.

Why This Detail Resonates

This isn't just a friendly reminder; it's an example of mistake-proofing, a design principle often called Poka-yoke. The system is designed to prevent inevitable human error before it happens.

• It externalizes the checklist. A user sending a campaign is thinking about copy, subject lines, and send lists. Forgetting a legally required footer is an easy mistake. Resend takes that cognitive load off the user and puts it onto the system, where it belongs.
• It builds trust through partnership. The feature says, "I've got your back." By preventing a serious mistake—one that could harm deliverability, annoy users, and even have legal consequences—Resend positions itself as a trusted partner, not just a passive tool.
• It provides the right friction at the right time. Instead of letting you send and then showing an error, it blocks the primary action. This momentary friction is valuable because it prevents a much larger, irreversible problem.

When to Bring This Detail to Your Product

This pattern is most effective for actions that are high-stakes, irreversible, or subject to "checklist fatigue." Ask yourself:

• Is there a common, costly mistake users can make right before a primary action? Think publishing a blog post without a title, deploying code with failing tests, or submitting a form with an invalid email.
• Is there a best practice or legal requirement that users might forget? This could be adding alt text to images for accessibility, including terms of service in a checkout flow, or warning a user they're about to delete a large amount of data.
• Can the system intelligently predict the user's intent? By anticipating the user's true goal (sending a successful email, not just any email), the system can provide proactive guidance.

Where You've Seen It

• Gmail: Famously scans your email for phrases like "I have attached" and warns you if you hit send without an attachment.
• GitHub: Prevents you from merging a pull request if there are merge conflicts or if required status checks (like automated tests) haven't passed.
• Vercel: Halts a deployment if the build process fails, preventing broken code from ever reaching production. It’s a server-side pre-flight check for your entire application.
• Figma: When exporting assets, it warns you if some frames have identical names, preventing you from accidentally overwriting files.