Change subject from mails after incoming

Hello,
in Emc 7 where can i change the subject from mails after incoming.
In EMC 6 i open the mail and then i can change the subject text direct and after click tab an messagebox ask to save the message.

In EMC 7 it is the same, but when the message is sign with a certificat, it does not ask and i cannot change the subject.
Info:
In EMC6 it runs.

in this moment i work so, that i drag and drop the message to the destop.
Than i change the sourcecode from the Mail and per drap and drop i copy the message to emc 7

Regards
Christian