the encryption normally is done with the public key of the recipient and the sender, so totally with 2 keys.
I wonder that in some cases a THIRD key is automatically used from my GNUPG keystore and I cant find any system for that. So for example, if I use “thomas” as sender and “fred” as recipient, the keys of thomas, fred and lisa are used for encryption, but the usage of "lisa"s keys makes no sense! How to disable this?
By the way it would be nice to see in a sent mail which keys were used for encryption. I only see the green and the blue bar in the sent email, but no informations of the keys.
The simplest solution is that you have extra configuration that adds the additional pubkey.
One may to get more details about the problem would be to use the command line
as indicated in https://wiki.gnupg.org/TroubleShooting and try the encryption
gpg --encrypt -vvv -r fred
and see how many pubkeys are used.
The problem appears in some cases in both OpenPGP and SMIME at the same time. The plugin suggests a second public key for encryption for the sender which makes no sense.
But maybe the reason is that I made a big amount of tests with importing and deleting public and private keys. It also seems to be a problem to delete keys completely.
If you get any new information in this matter, it would be nice to hear from you.