DKIM (DomainKeys Identified Mail) is a possibility to make an email sender more easily identifiable for the email service providers (ESPs) who receive and filter email. The message is identified by the domain it is sent from, that is, the part of the address after the @.
If the delivery address differs from that that has been registered with the DNS server (Domain Name Server), there is an increased likelihood that the email will be classified as Spam by the email service provider.
Why it is needed?
It is because ISPs like yahoo, gmail, hotmail dictate it! Look at their bulk mailing guidelines, which must be followed by all email marketers and their email marketing solution providers.
If your bulk or mass email marketing solution provider doesn’t provide it, it is time for you to ask for it. If they are unable to do it, look for other email service providers who can provide it.