Is It a Real Business Email?
A professional email uses the company’s own domain, like name@company.com, not a free account like company.support@gmail.com. Real marketplaces, banks and government offices never write to sellers from Gmail, Yahoo or Outlook addresses, so the checker warns you when an “official” email comes from one.
For a business domain, it also shows how long the domain has existed, whether it can receive mail, and whether SPF and DMARC are set. An established business usually has all of these; a new domain with none is a warning sign.