KMail 2 doesn't allow entering separate subkeys for signing and encrypting but only allows the use of the primary key for signing and encrypting. Even editing .kde/share/config/emailidentities directly and explicitly setting the fingerprints for the signing and encryption subkeys doesn't work: PGP Encryption Key= & PGP Signing Key= will both be reset to the primary key. Reproducible: Always Steps to Reproduce: 1. Attempt to set separate signing and encryption subkeys Actual Results: Only primary key is selectable. Expected Results: Being allowed to use separate signing and encryption keys.
If there is a subkey available for a certain operation then the mainkey is never used. Have a look at the results with gpg --list-packets. You will most probably see that the subkeys have been used. If there are several equivalent subkeys available then you are in the situation that you cannot choose. But this is a seldom, rather complicated operation and probably not your problem. It would be nice, though, if KMail could be configured to use a separate GnuPG configuration. This is possible for KGpg. But that would solve only half of your problem and would not be a very clean solution.
This bug has only been reported for versions before 4.14, which have been unsupported for at least two years now. Can anyone tell if this bug still present? If noone confirms this bug for a Framework-based version of kmail2 (version 5.0 or later, as part of KDE Applications 15.12 or later), it gets closed in about three months.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.0 aka 15.08), please open a new one unless it already exists. Thank you for all your input.