To list secret keys, you use:
$ gpgsm –list-secret-keys
It was found that after importing, no secret keys were listed. This was due to a problem with the GPG_AGENT_INFO variable that pointed to seahorse, a key manager. To fix this problem, do:
$ export GPG_AGENT_INFO
Also, remove seahorse*.rpm files from your system.
This did not work still in Claws Mail, but in Thunderbird, S/MIME worked very well.