DomainKeys support in H-Sphere has certain limitations.
Outgoing messages are signed with Domain Keys only if:
- The mailbox in question has DomainKeys check enabled;
- skipcachk/skipsachk are set to "Default settings".
Even if those conditions are met, e-mails that are send out by sendmail program that is used in Horde by default will not be signed. Please use SMTP server to send signed e-mails.
To check what program is used by Horde and how to switch to external SMTP server please check the H-Sphere System Administrator Guide chapter Setting SMTP Server for IMP.