What is DKIM?
Read the article here.
How to set the DKIM?
Settings on your server:
To properly configure DKIM for your domain, you must add the following DNS record:
emaillabs._domainkey.domain.com. CNAME emaillabs._domainkey.emaillabs.co.
where domain.com is the name of your domain.
Settings of the EmailLabs panel:
In the “Enter the custom domain”, enter the domain from which emails will be sent.
However, when the dispatch is directed by a larger number of addresses, select “Download dynamically from the field FROM”, then your domain name will be downloaded automatically from the sender of the e-mail.