There is a better way!

[ddavisso4]

FYI, investigated what I perceive is a better way to actually do the signing.
https://support.microsoft.com/en-us/help/2636465/how-to-sign-or-encrypt-mail-messages-programmatically

Here's some code:

string PR_SECURITY_FLAGS = "http://schemas.microsoft.com/mapi/proptag/0x6E010003";
int currentSecurityFlags = mailItem.PropertyAccessor.GetProperty(PR_SECURITY_FLAGS);

int updatedSecurityFlags = currentSecurityFlags;
//newSecurityFlagSettings |= 0x1;   //Enrypt
updatedSecurityFlags |= 0x2;     // Sign

mailItem.PropertyAccessor.SetProperty(PR_SECURITY_FLAGS, updatedSecurityFlags);
mailItem.Save();