S/MIME is natively supported

app/src/main/java/eu/faircode/email/FragmentCompose.java

@@ -3401,8 +3401,11 @@ public class FragmentCompose extends FragmentBase {
 
                         // Encryption
                         if (ref.ui_encrypt != null && !EntityMessage.ENCRYPT_NONE.equals(ref.ui_encrypt)) {
-                            if (ActivityBilling.isPro(context) && Helper.isOpenKeychainInstalled(context))
+                            if (ActivityBilling.isPro(context))
-                                data.draft.ui_encrypt = ref.ui_encrypt;
+                                if (Helper.isOpenKeychainInstalled(context) ||
+                                        EntityMessage.SMIME_SIGNONLY.equals(ref.encrypt) ||
+                                        EntityMessage.SMIME_SIGNENCRYPT.equals(ref.encrypt))
+                                    data.draft.ui_encrypt = ref.ui_encrypt;
                         }
 
                         // Reply template