A catch-all (or "accept-all") domain is configured to accept mail for any local part, then sort out delivery internally. From the outside, every address looks valid — which makes the standard SMTP probe useless.
Popular consumer domains
Large consumer providers — including mail.ru, yandex.ru, and gmail.com — either behave as catch-alls at the SMTP layer or otherwise refuse to reveal whether a mailbox exists. A plain RCPT TO check would mark a typo and a real account identically.
CleanContact handles the most popular domains with provider-specific checks tuned to each major mailbox. That lets it tell a real account from a non-existent one on the addresses you see most — mail.ru, yandex.ru, gmail.com and the rest — and return a real Good or Bad where a generic SMTP probe could only shrug.
Why this matters for your list
- Without catch-all handling, every address on these huge consumer domains would come back as a meaningless "valid".
- Provider-specific probes turn an unverifiable Unknown into a real verdict.
- It keeps your bounce rate honest even on the domains that try hardest to hide it.
Because these checks are heavier, they run on their own queue with dedicated per-node cooldowns so they never slow down ordinary SMTP validations.