Some domains are configured to accept every email sent to them, whatever comes before the @. That's a catch-all. It means an email verifier can't tell whether lena@ or l.park@ is a real inbox, because the server says yes to both.
Sending to catch-all addresses is riskier than sending to verified ones. The mail may be accepted and then quietly dropped, or bounce later, and you won't know whether anyone read it.
For important prospects on catch-all domains, look for the address published on the site itself, in the author bio or contact page, rather than guessing a pattern.
Example
A verifier returns "accept-all" for every address at a small blog's domain, so Margin uses the address shown on the blog's contact page instead of a guess.
Putting it into practice
Where to use it: Email permutator and Email deliverability for link outreach.