DKIM, also known as DomainKeys Identified Mail, is a form of email authentication that digitally signs emails to prove that they were sent from authorized domains and aren't fake emails. Companies like Google and Yahoo use DKIM to sign their emails and it is becoming ever more popular.

To be able to do this yourself, you'll firstly need to download a DNS server, like Simple DNS. Once you've downloaded and set it up, click the records tab and login. On the left side of the screen, you find the domain you mean to assign DKIM to and select it. On the right side, right click and select New TXT-record.

Now you need to generate a public key to put into the new TXT record. To do this, search for a DKIM key generator, like dkimcore.org, and enter the domain you want to put DKIM onto. This will generate a public key and a private key, and also a selector. The selector will look something like 1443607874.<domain name> With the public key, copy the key and paste it into the text data box on the DNS server. Remember to remove the speech marks and any spaces in the key. Just above the public key there will be a code that looks a bit like 1443607874.<domain name>._domainkey.<domain name>.co.uk. Paste this into the Record name (host) box. Then click OK.

Now you need to sort out the private key. Copy the private key and paste it onto a word document, like notepad. This document needs to be saved into the VPOP3 directory, so save the document as domainkey_<domain name>_<selector>.key and save it to the VPOP3 directory.

Now go onto VPOP3 and click the Services tab, and SMTP Server, then click the spam reduction tab. Once there, change the DKIM Signing box to All Local Senders, and add the selector to the DKIM Selector box. Then click Submit.