If you want to allow Salesforce to send email on behalf of your domain, setting up DKIM in your DNS is essential. Your email deliverability is directly impacted in case you skip the process of configuring DKIM. Moreover, the chances of your email being marked as spam also increases. This article will briefly cover the steps required to be followed to successfully setup your DKIM for Salesforce.  

• Steer to the Salesforce Setup menu, type DKIM Keys in the Quick Find box, and then select DKIM keys.

• Now, click on Create New Key

• Select the RSA key size. Consider email recipient limitations and industry-specific security regulations when choosing the key size.

• Enter a unique name for selector

• Next is the alternate selector, this allows Salesforce to auto-rotate your key. Enter a unique name for alternate selector

• Enter the name of your domain

• Now, select the type of domain match you want to use

• After entering all the details, click on Save to record your entries. 

Salesforce will now publish TXT records to DNS. Your CNAME and alternate CNAME records appear on the DKIM Key Details page when the DNS publication is complete. It can take time for DNS publication to finish. 

• Publish the CNAME and alternate CNAME to your domain's DNS.

• Once done you can click on Activate on the DKIM key details page. 

You are now done with setting up DKIM for Salesforce. You can refer to this link for more information regarding DKIM for Salesforce.