An MX record is the DNS record naming a host that accepts mail for a domain, with a preference number that orders the candidates — lower is tried first. A domain may publish several, and the set of them is the first thing any verification reads.
What Spaw answers
mx_found says whether a host that can accept mail was found. mx_provider names the operator behind the hosts, recognised from their patterns — Google, Microsoft, Proton, Fastmail, Mimecast and the rest each have a page under email providers.
Three reasons come straight out of this step: no_mx_records when DNS returns neither an MX nor an A record, mx_unresolvable when every published target fails to resolve or points somewhere that cannot receive mail from the internet, and null_mx when the domain publishes the standard "no mail here" record.
What it cannot tell you
An MX record is a claim about the domain, not about a mailbox. It is checked live on every lookup rather than read from a cache, because DNS is the one part of the answer that costs nothing to get right and changes without warning.