The sender challenge, and what it costs

An option, not a default

Some filtering services reply automatically to unknown senders: "confirm you are a person so your message gets through". Cybelia offers this mechanism, called the sender challenge, but it is switched off by default — and this lesson exists to tell you why before you turn it on.

What it brings

It is very effective against bulk automated sending: a robot never confirms. An unknown sender's first message waits, and if they confirm, their address is remembered — no further challenge afterwards.

What it costs, in three points

Machines do not confirm. Invoices sent by accounting software, order confirmations, your website's forms, your bank's notifications: none of them will click. Those messages will not arrive. So those senders must go on your allowed list before you turn the option on — not after three weeks of wondering why an invoice was missing.

You move the effort onto your correspondent. A client writing to you for the first time gets a request they were not expecting. Some confirm, some find it suspicious, some simply forget. A first sales contact can be lost there.

The challenge reveals that the address exists. That is information a malicious sender can use.