To my ongoing aggravation, it appears to not currently be possible to use S/MIME with WP8.1 or W10M unless you are using an Exchange ActiveSync-based account. Secure email on IMAP and POP3 accounts is not supported, for some reason. From the user's perspective, this is a completely pointless and artificial restriction - other platforms' email clients have no such limitation, including Outlook on PCs - but I'm sure Microsoft thought they had a good reason.
You should go into Microsoft's
Insider Hub /
Feedback apps (any possibly also relevant UserVoice sites you can find, but it looks like those have been shut down for Windows Phone) and upvote the (many) suggestions requesting S/MIME support for non-Exchange email accounts.
The following instructions cover enabling S/MIME on Windows 10 Mobile (W10M), using the current Insider Fast build. Older versions of the OS may have different steps (you need to have at least Windows Phone 8.1 Update, I think). If you connect to an Exchange server (usually means a school or business email address), you should be able to make S/MIME work on the phone. Even without that, the option is probably present (but disabled):
- Open the email client (
Outlook Mail on W10M).
- Tap the three dots in the lower right to show the application menu.
- Tap on
Settings to open it. (For WP8.1, this is roughly the same as going to the
Settings hub, swiping to the
Applications pivot, and choosing
Mail, I think).
- Tap on
- Select an account at the top (only accounts using ActiveSync will be available).
- Turn on "Always sign with S/MIME" and/or "Always encrypt with S/MIME", as you please.
- Leaving the certificate selection on "Automatically" should be fine, but if you tap "Manually" you can see, in the new drop-down that appears, what certificates the phone knows about that could be used.
- Once you've made your desired changes, use the Back button to exit back to the email client.
To check if it's working, do the following:
- Create a new email (or reply to / forward one) using the relevant account.
- If the mail is signed, a little ribbon outline should appear on the right side of the Subject line. If the mail is encrypted, a little padlock icon should appear in the same area.
- To toggle encryption or signing, tap the three dots in the lower-right to display the message composer menu.
- Tap on
Options, then on
Encrypt to toggle those options.
- Hit Back or tap on the three dots again to close the menu.