The form says sent and you never get anything

This one quietly costs the most money, because nothing looks broken. The customer sees a thank-you, you see an empty inbox, and neither of you knows. I have found this in production sites that had been live for months.

1. The success message is not connected to anything

The most common version: the code shows the thank-you first and sends the request after, or shows it regardless of the answer. Any failure becomes invisible.

Check what the server returns when it silently drops a message, for example when a spam trap triggers. That answer must not read as success.

2. The spam trap is eating real people

Hidden fields that bots fill are standard. If the field is named something a browser recognises, like website or company, autofill will fill it for a real visitor and the message is thrown away as spam.

I have shipped this bug myself. It is invisible unless you test with autofill switched on.

3. The email provider is silently rejecting it

Free tiers stop, domains go unverified, and providers refuse mail that claims to come from an address you do not own. Many APIs return a failure your code never reads.

Sending from a bare address on a domain you do not control is the usual reason mail disappears without a bounce.

4. Nobody has ever tested the whole path

The form was tested when the code was written, then the API key rotated, or the endpoint moved, and no one submitted a real request again.

If your site takes orders, put a reminder in your calendar. A form that breaks silently in July is discovered in September.