What is DKIM?
How signed messages help recipients validate your sender identity.
Start with the basics
DKIM adds a cryptographic signature to outgoing email. Your provider gives you a selector and DNS record.
What to check
The selector is the first part of a name such as selector1._domainkey.example.com. It is normal for providers to use different selectors.
A safe next step
Verify the published record and check that the sending provider is actually signing messages.