Vibecoding your own email marketing tool? The send button is the easy part.

Yesterday, I saw Tyler Denk posting this on X. And it inspired me to publish something about vibecdoing your own app here.

Here's just like the top 5-10 things:

• Auth isn’t optional anymore. Gmail and Yahoo bulk-sender rules require SPF, DKIM, and DMARC aligned on your sending domain. Miss one and you’re junked or rejected.
• One-click unsubscribe. Bulk senders need a List-Unsubscribe header with one-click support, and you have to honor it within days. That means a suppression list that every send actually checks.
• Spam complaints under 0.3%. Go over that and Gmail punishes the whole domain, not just the campaign.
• Bounces and complaints need webhooks. If you don’t process them and auto-suppress, you keep mailing dead addresses and your reputation tanks.
• Throttling and warm-up. Blasting 50k emails from a fresh domain or IP on day one is the fastest way to get blocklisted.
• Rendering. Your template looks great in Chrome and breaks in Outlook. Dark mode inverts your logo.
• Leaked keys. We’ve seen vibecoded frontends ship the sending API key straight to the browser.

None of this means don’t build it. It means the sending layer is the hard part, so don’t vibecode that piece. Put a real sending API underneath, and test every template and flow in a sandbox before it ever touches a real inbox. That’s literally why we built Mailtrap’s Email Sandbox and sending infrastructure.

Anyone here actually running a homegrown email tool in production? What broke first?