DKIM is a combination of a SmarterMail setting and TXT record. The setting also needs to be enabled by Support. If a request hasn't already been made, open a ticket with the Support Department
First log into the SmarterMail web interface as the domain administrator (default is postmaster). If the web interface address or administrator email address and password isn't known, log in automatically at: DiscountASP Control Panel > SmarterMail Manager > SmarterMail Management Login as Primary Admin
Once logged into the webmail interface, click Settings (gear icon), expand Domain Settings, expand Advanced Settings, click Mail Signing and finally click the Certificate tab.
Selector: A string of characters to create the TXT Record Name. Anything can be entered, so in the example above the date is used.
Key Size: It would be best to leave it as a 1024 bit key.
After entering a value into the Selector field, click the Generate Key link to create the TXT Record Name and TXT Record Value. Those records will be added to your DNS to enable the DKIM.
If hosting DNS through DiscountASP, in a separate browser tab navigate to the DNS Manager: DiscountASP Control Panel > DNS Manager > ("I understand and agree" if prompted) > the domain name link > TXT Record Manager > New
Domain Name: Everything preceding ".domain.com" should be copied and pasted into the field. In the example below, it would be: jan2019_domainKey
TXT Data: The TXT Record Value is copied and pasted here.
After the TXT record has been added, it may take an hour or two for the DNS propagation to complete. Which can again be reviewed at https://www.whatsmydns.net/. Afterwards, the Test DNS button in Mail Signing should return "DNS test was successful".
Testing SPF and DKIM
A simple test of the SPF and DKIM can be performed by sending an email to a Gmail recipient and checking the email header. After the email is received, view it, click the three vertical dots for more options then Show Original.
If set up properly, there should now be two PASS notifications on the Original Message page.