Since updating to iOS 11 and renewing my SMIME certificates they've stopped working for encryption.
Please see (Dutch) screenshots for the valid certificates and the grayed out options to choose "Encrypt" and "Sign".
What options do I have, because the free Comodo service offers no solution. You can get the certificates (in IE only) via https://secure.comodo.com/products/frontpage?area=SecureEmailCertificate
Extra steps: Download the root certificate, because if you don't the certificate will be red "Untrusted".