@ KaedeX503, for what was described here and observed the problem affects the following items:

1 - You send a message to a user and when he replies it lists the original topic and the "Re" for reply but it says the message come from original sender.

2 - If you try to reply this message you will only send the message to yourself, so you need to start a new message to the people you want to send.

3 - If you receive messages for different people you cannot say who sent it to you. (I hope that when YouTube correct this problem the old messages will be corrected to reflect who sent it)

4 - If a user just use the reply to answer a message, it will not be aware that the receiver doesn't know who replied to him.

5 - @YT Kendall, stated that any of this decribed problems are related to hijacked accounts, and that YouTube is working in a solution.

Please YouTube some sort of alert is necessary, at least in the case of problem 4, or the message system will be, if not already, a house of angry users don't receiving replies for their messages.