Let's assume my professor sends a signed but unencrypted (S/MIME) e-mail to us students like this:
there will be no class tomorrow. I'm sick.
Now I could simply replay this mail next month to my fellow students and my professor would be the only one showing up in the class room. All the other students would believe this mail is valid, since it is still signed (by my professor) and sent via a faked sender address.
My idea is, that this e-mail should have been addressed to a specific recipient ( Dear student A.. ). This way each recipient can simply proof via plaintext, if this mail is addressed to him/her.
Is it correct, that sending trivial signed but unencrypted (or encrypted-then-signed) mails like this should be avoided, since it could be turned against you?