DKIM check
Find the domain's DKIM keys and see whether they're strong enough.
The selector is the name your mail provider gave the key, for example google for Google Workspace or selector1 for Microsoft 365. It appears as s= in the DKIM-Signature header of a sent email. Without one, the most common selectors are tried.
What DKIM is
DKIM (DomainKeys Identified Mail) is a digital signature your mail server adds to every message it sends. The matching public key is published in DNS, at selector._domainkey.yourdomain.
The receiver checks the signature with that key. A valid signature proves the message really came from your domain and wasn't changed on the way.
Keys are found by their selector, which can't be listed from DNS. That's why a check without a selector tries only the common ones.
This checks the record in DNS only. To see how a real message from your mailbox is signed and judged, use the complete email delivery test