not in the terminology of evolution. aliases are other email identities for the email account I have credentials to.
I’m afraid having a per-alias option for Security is too complex, which means, in your use case, the only option is to use the old way of defining account aliases, which means to configure a send-only account,
this does not apply in my case since i do not want to send mails neither with email@example.com nor with firstname.lastname@example.org. I want to encrypt mails for them, and send it to them. A send-only account does not help here.
the issue is, that evolution selects the (public) encryption key according to the recipients email address. However, there are situations I have a public key for the recipient as a person, but not for the email address specified in the public key of the recipient. Or am I wrong? Is the solution obvious? Then I would be very happy for detailed explaination.