default smime key for signing

Hi 

I had to upgrade my X509 Smime certificate I also use for
signing/encrypting email.

Now I have a bunch of X509 certificates all with the same email address.

Could someone remind me how to tell epg (which I am sure I use) which
key to use (either by the key id, or fingerprint)

gpgsm is set up to sign from the command line using the new certificate,
but some emacs fail to recognise this

Thanks 

Uwe Brauer