DKIM Signature Verification for Outgoing Emails
Assuming you have already created your DKIM data, added them to your DNS, and want to verify your DKIM configuration for signing outgoing emails. The DKIM record is stored in DNS as TXT and you can manually check the DKIM record for a domain by using nslookup or dig as follows
1. In the terminal type
nslookup -type=txt yourSelector._domainkey.yourDomain
dig txt yourSelector._domainkey.yourDomain
Replace yourSelector with your DKIM selector and yourDomain with your domain name, for example,
nslookup -type=txt 0E9F184A-9577-11E1-AD0E-2A2FBBAC6BCB._domainkey.example.com,
2. If a DKIM record exists, the result would contain your record starting with
3. If a DKIM record doesn’t exist or there is a problem retrieving the record for the domain there would be no
v=DKIM1 property in the output.
To test your DKIM, you can also check if the DNS entry for your DKIM is correct by entering your domain and selector into https://mxtoolbox.com/dkim.aspx. If you set your DKIM entry correctly it will show you the results meaning your DKIM passed the test. The mxtoolbox should be enough to test your DKIM but there other tools available online for this purpose. For example, another tool you can use is http://tools.wordtothewise.com/dkim.
You can also send an email to Gmail and to see if your email shows Signed by on the email information, which means your email DKIM signature has been successfully recognized by google.