iOS S/MIME Signing Issues When I try to send an email using an installed S/MIME certificate on iOS, I get an alert that says: 


  Unable to Sign: You can't send signed messages because a signing identity for the address could not be found. Go to the advanced settings for this account and choose a signing identity.


I have a signing identity chosen in the S/MIME settings for the account, and I have tried exporting my private key from my Mac's keychain many different times. The certificate/key match the address I'm sending from. Am I doing something wrong?
Not all certificates are supported by iOS, it depends on the encryption method.
I had issues with the free Comodo S/Mime certificates and switched to the free S/Mime certificates from Startssl.com, they work fine