DKIM takes email sender identification a step further by associating a domain name and owner to the content of the email message, allowing the organization to vouch for the content of the message. This is accomplished by cryptographic signing of the content.
DMARC improves on SPF and DKIM by giving sending organizations a stronger means of communi cating the confidence of their SPF and DKIM implementations and by providing a framework for receiving organizations to provide feedback to sending organizations, including detailed information on who is attempting to spoof sender domains.
Mail-Tester.com - SPF / DKIM Testing tool
Check if your domain has these 2 email signatures set up and valid.
What's DKIM and SPF? They're 2 effective email signatures against spoofing, phishing or impersonation. When recipients receive your emails, their spam filters automatically poke your domain to see if those signatures are not forged.
dkimvalidator.com - Validates DKIM signatures
This is useful for verifying the end-to-end functionality of your DKIM and SPF configuration, as well as your actual SpamAssasin score. Since your email is sent through regular mail servers, all of the headers that these tools see are the same as your email recipients will see.
MXToolbox.com - DKIM E-Mail checking
The DKIM Record tool will test a domain name and selector for a valid published DKIM key record. DomainKeys Identified Mail (DKIM) defines a domain-level digital signature authentication framework for email by permitting a signing domain to assert responsibility for a message in transit. DKIM authenticates the reputation and identity of the sender and their email signing practices for additional handling such as delivery.
dmarcanalyzer.com - DKIM E-Mail checking
DKIM (DomainKeys Identified Mail) is an email authentication technique that allows the receiver to check that an email was indeed sent and authorized by the owner of that domain. It works together with DMARC (and SPF).
toolbox.googleapps.com - Google Apps / Gsuite E-Mail checking
If your company uses Google Apps or GSuite for your email, you can check your configuration using this link. Excellent Tool! A must of have for GSuite SysAdmin's.