KYC is the identity-verification process, mandatory for regulated financial services, that many messaging providers also impose on senders: ID documents, company registration and a review before an account can send.
Know-your-customer rules exist in banking and payments law to prevent money laundering. Messaging is not a regulated financial service, but most providers adopted similar checks to protect their carrier relationships: passport scans, company papers, use-case questionnaires, sometimes a call with sales. The process filters by paperwork rather than by behaviour, and it excludes sole traders, teams in countries with slow registries and anyone who does not want a vendor holding their identity file.
What the law actually regulates in messaging is content and consent. A provider can enforce those without knowing who you are.
At SMSMeteor: there is no KYC: an email address opens the account, and the acceptable use policy governs what is sent. See what no-KYC means here.