Encrypted e-mail using a certificate not issued for that e-mail

Solution 1:

That is not possible. It is the certificate that specifies the email address it can be used with. The allowed email address is in the email field, as well as in the rfc 822 name field, where more than one email address can be given.

It does not make sense to allow the sending client to use a certificate for email addresses that are different from those allowed by the certificate, because that presumes knowledge about internal alias mappings on the recipient side. If the recipient could and would want to accept emails encrypted with the certificate to any additional email addresses, they should have been listed in the certificate.