Evan, are you still having this issue? I cannot duplicate it. I have tested setting enablemailsigning to both true and false via the domain admin web service call, SetRequestedDomainSettings. I verified this by both calling GetRequestedDomainSettings and examining the domainConfig.xml. Also, if I edit the domain as a system admin, under Features the Mail Signing is unchecked. When this is set to false, in the web UI under DomainAdmin | Settings | Domain Settings | Advanced Settings, the Mail Signing option should not appear.