About the MX Record Lookup
The MX Record Lookup shows which mail servers accept email for a domain, in the order sending servers will try them (lowest priority first). Combine it with the SPF Checker and DMARC Checker for a full deliverability picture.
Lower = preferred. Sending servers try the lowest-priority MX first and fall back to higher numbers if it is unreachable.
Possibly: without MX records, mail falls back to the domain's A record. But for any serious mail setup, explicit MX records are expected.
Hosted email (Google Workspace, Microsoft 365, mailbox.org …) publishes its own server names as your MX — that is normal and correct.