Follow these steps to set up DKIM Signing for your domain:

Log into SmarterMail as a Domain Administrator.
Click on the Domain Settings icon.
Click on General in the left menu.
On the Email Signing card, click Enable.

SmarterMail will display a unique Text Record Name and Text Record Value.

Add the TXT record to your DNS server using the provided name and value, or contact your DNS provider to handle this for you.

After the TXT record has been added and propagated to DNS, click on the Enable button once again. SmarterMail will attempt to verify the DNS settings and DKIM Signing will be enabled.

To adjust the mail signing settings, click the Settings button. Note that in most cases, these settings should not need to be altered. However, if you are familiar with DKIM processing and would like to specify how closely you want the system to monitor messages in transit, the Body Canonicalization, Header Canonicalization, Header Field to Use and Header Fields can be adjusted. More information on these settings can be found at https://dkim.org/specs/rfc4871-dkimbase.html