See also other Zimbra pages in this wiki
https://tipstricks.itmatrix.eu/configuring-dkim-authentication-in-zimbra/
http://wiki.zimbra.com/wiki/Configuring_for_DKIM_Signing
http://files.zimbra.com/website/docs/8.5/Zimbra_OS_Admin_Guide_8.5.0R1.pdf
If this is the first DKIM-enabled domain on the Zimbra Server, you need to reboot the server after configuring the first domain.
DKIM is configured on a per-domain basis. Change the domain name to your own domain in the following command.
su - zimbra /opt/zimbra/libexec/zmdkimkeyutil -q -d mydomain.com #Query DKIM settings for a domain /opt/zimbra/libexec/zmdkimkeyutil -a -d mydomain.com #Add DKIM settings for a domain
TXT
field in the DNS of domain you are configuring. For Virtualmin, do a manual edit and paste the results of the above command.
After configuring DNS, test the DKIM key (use your own domain name and selector):
/opt/zimbra/opendkim/sbin/opendkim-testkey -d mydomain.com -s 0E9F184A-9577-11E1-AD0E-2A2FBBAC6BCB -x /opt/zimbra/conf/opendkim.conf
A null result indicates the key is correct.