According to a definition given by my prof:

Message authentication methods verify that the message is from the right sender, however they dont guarantee non-repudiation.

To me this in contraddition, what does "right sender" means ? Can you explain to me ?

According to a definition given by my professor:

Message authentication methods verify that the message is from the right sender, however they don't guarantee non-repudiation.

To me this sounds like a contradiction, what does "right sender" mean?