Steps to reproduce: send a s/mime encrypted mail and after that disable s/mime in the GpgOL settings.
Click on and decrypt the s/mime mail in the sent folder. Change to another mail. Change to the s/mime mail again.
-> mail is decrypted
Expected: message that the mail can not be decrypted.
aheinecke says cause is 91c86d7cd53a190fa73f1b62372d74e331887a1c