Hi
I have exactly the same issue.
I've created a S/MIME certificate at both Comodo and StartSSL.
Both certificates are imported, and in my outlook account settings, I can choose either of them for signing/ciphering.
These certificates work well on Thunderbird (Linux) and "even" Outlook on my Surface 2.
But :
a) signed emails have nothing particular when received ; I can't even see the attached signature. On a IMAP account, the attached file is visible
b) even forced, I can't sign.
People say it works here for instance:
S/MIME mit Windows Phone 8.1 | Thoms Blog (I don't read German but the captures are quite telling). Note that it looks like 8.1, not GDR1 (he has no "automatic/manual" option).
I wonder how on earth can we use S/MIME on WP. No OpenPGP, no S/MIME support for individuals, is this how MS intend to "help us master our privacy"?
Any hint/tips appreciated.