As one of the most popular email authentication methodologies, it works by using cryptographic technology that adds a digital signature to your message header. This DKIM signature validates and authorizes your domain name in the eyes of the receiver. The DKIM signature is created using a unique string of characters stored as a public key.
SendGrid will always provide you with a custom DKIM signature. However, your custom DKIM signature is only automatically updated if you select automated security when creating your domain whitelabel. If you turn automated security OFF, you will be responsible for updating your DKIM signature whenever you make a change to your sending domain.
In order for you to add a DKIM to your domain, you just need to go to your Sendgrid account, next choose Settings and go to Address Whitelist
Then just copy the following records to your Hosting provider, in the DNS manager of the particular domain that you want to Whitelist
In order to configure DKIM with SparkPost, just go to your SparkPost account and select the account button, on the left panel, from there go to Sending domains, click add a domain.
And you now have a fully configured sending domain with SparkPost.
If you are using a very own custom SMTP then you need to install a DKIM Certificate on your server using this link https://port25.com/ and then continue with the DNS settings configuration.
Here are some tutorial links on how to configure DKIM with these specific DNS providers
Configure DKIM using NameCheap
Configure DKIM using PLESK
Configure DKIM using Webmin/Virtualmin
Configure DKIM using VestaCP
Configure DKIM using DirectAdmin
Configure DKIM using PostFIx
Configure DKIM using EXIM