Enabling Client Certificates for iOS Mail
Enabling these options will allow you to digitally sign and encrypt all email sent
from your device.
Settings > Mail, Contacts, Calendars.
Select the email account associated with your Client Certificate.
Accountbutton with your email address.
Accountscreen, change the "S/MIME" setting to
ON. The "Sign" and "Encrypt" options are off by default.
To enable signing, tap
Sign, and then select your certificate. This will digitally sign every email you send from your email account on this device.
To enable encryption, tap
Encrypt, and then select your certificate.
Note: The encryption option will attempt to encrypt all correspondence from your iOS device. If you do not have the public certificate for a recipient, the email message will not be encrypted.
- To enable signing, tap
You also read Apple's knowledge-base article related to this here: