This is prevented by simply, using proper human communication.
Normally, you begin a message with like:
Nice to hear from you again.
bla bla bla"
Thus Carol, would easily to detect this disrepiancy since the message is not adressed to him/her, and would of course ask Alice "Did you send this to me? Then you have the wrong adress." and Alice would say "No, I didn't send it to you, the message must have been leaked and replayed."
Note that the "Hello Bob" should be part of the signed portion, and thus is inmodifiable by the attacker.
If you don't know the name or intended recipient of the mail, then its normal to make the content stand out in a specific way to make it clear that its intended for someone specific, so if the message lands in the wrong inbox, the receiver will understand the message was not intended for her.
A example, could be if you are sending a message to "support@random_isp.com"
My broadband account just got suspended, my RandomISP customer number is 153424.
Could you check my account details if the latest bill payment has arrived and whats happened if not."
If this mail would land in your inbox, it would be clear that its not intended or sent to you.
Also note that theres other systems like SPF and DKIM, which will verify and prevent the usage of the sender's email adress, which would in this case also give away any replay attempt, since the sender email would not match the public key that correspond to the private key the mail was signed with.