Are attached keys sent until an encrypted email is received?

We used to rely on 'sign_used' to know if we got an encrypted email as signatures were only checked if emails where encrypted, but #7438 (closed) seems to imply that now we verify signatures always. Can we fix when and how we attach the public keys to our emails?

(from redmine: created on 2016-04-12, relates #7438 (closed))