Why MX matters for cold email
MX records say where email to your domain goes. A domain you send cold email from must receive the replies: without MX records, or with a null MX, every answer bounces, and receivers distrust domains that cannot be written back to.
The lowest priority number is tried first; the others take over when it does not answer.
Questions
What are Google Workspace's MX records?
Today Google asks for a single record, smtp.google.com with priority 1; older set-ups list aspmx.l.google.com and its alternates. Both work.
What is a null MX?
A single MX record pointing to ".": the domain says it accepts no email. Never send cold email from such a domain.
I just changed my MX records. Why does the checker show the old ones?
DNS changes take time to spread, up to the record's TTL. The checker also keeps an answer two minutes; check again a little later.
Where Coolie comes in
A domain that passes these checks is the start. Coolie then sends from your own mailboxes and LinkedIn accounts, paced over working hours, with daily caps per account and per action, a pause when someone replies and an unsubscribe line: how Coolie protects deliverability.