I’ve been trying to get my s/mime email signing certificate to work under ios5 and i think i’ve found a bug.
If you are trying to send a signed email from your iOS5 device and instead your recipients are receiving an attachment called smime.p7s it is likely that you are using gmail or google apps and are also using the exchange connector in iOS.
My guess is that Google didn’t setup a PKI infrastructure properly and when iOS tries to go a fetch a certificate from the non-existent exchange server the process fails and then an attachment is created.
If you setup your iOS mail account to google via IMAP then smime works as expected.
Filed a bug report with apple – 10302200.
[Update: 7 March 2012 – This is not fixed in iOS 5.1]
[Update: 17 November 2012 – This is not fixed n iOS 6.0.1]