Messages are put into wax-sealed envelopes before being sent over the internet. This evidence is an open envelope with some wax on it.

This digital signature proves the message was written by a party to the communication, but it doesn't prove which one.

Encryption keys can be used to make digital signatures. Anyone with the key for this communication could've made that signature; not just the sender, but the receiver as well.

(I'm sure you can think of more.)