Same problem here, Felipe.
There is some info here http://support.apple.com/kb/HT4977
It seems we have to somehow include our digital certificate in a server. But which server?
Some say our sertificate must be in the GAL (global address list). But I still don't know how to do it.
I'll keep trying.
Any client that does not support SMIME shows a .p7s file attachment. GMail web interface is one such client that does not support SMIME.
Penango is a cool service that adds a local firefox extension that adds SMIME support to GMail.
Outlook supports SMIME, so between iOS Mail and Outlook SMIME will recognize correctly.
I thought so too but even when I send an email from my Exchange account using my iPhone, to myself, and I only open it with Outlook, the attachment is still there and the message is not certified. When I send it from my Outlook to my iPhone though, then it is certified. So it is just messages coming out of my iPhone to any other account or client, be it S/MIME compatible or not, the message is not certified as it should be.
I'm still in the dark about this...
Felipe, got it!
Take a look at:
See section “Sending Encrypted Email Without Exchange”. Worked perfectly for me.
Meanwhile I've found a workaround, you can read at my thread here:
The point is: there's a bug with configuring mail as an Exchange mailbox and SMIME support on iOS 5.x.x, in my situation, the workaround was to pick up an additional mailbox for the same mail account and configure it as a gmail mailbox, then set is as default sender mailbox. My account is a google account which is managed by my own domain with google apps. If the account is configured as a gmail mailbox, the signature is recognized by Outlook and iOS clients as well.