The mechanism, briefly
Standard verification works by asking the receiving server whether it will accept mail for a specific address. On most domains the answer distinguishes a real mailbox from a made-up one. On a catch-all domain the server has been told to accept everything and sort it out later, so the same question returns the same positive answer regardless of what you ask about. Nothing is broken; the check simply cannot see anything.
Why it matters more than it used to
Mailbox providers now enforce hard limits on how many complaints and bounces a sender can generate before delivery is throttled or blocked. An unverified batch no longer just wastes budget, it damages the sending domain the rest of your outbound depends on. Because catch-all addresses pass a naive check, they are the addresses most likely to slip into a first send.
How Prospectal treats them
Prospectal does not return catch-all as a result. Every email comes back Valid or Invalid. An address is marked Valid only when verification confirms it; an address on a catch-all domain that cannot be confirmed is marked Invalid. You never receive a separate pile of uncertain addresses to judge before you send.
The practical rule
Send to the addresses marked Valid. Keep the ones marked Invalid out of cold email, including those on catch-all domains, and if a person there is commercially important, reach them on a channel where a failed attempt costs nothing.
Put it into practice