Manual:$wgNoReplyAddress/en

Details
This address is used for the Reply-To: header of automatically generated emails. Any email replies will be sent to this address. It needs to be a correctly formatted email address, but it doesn't need to be real in any way.

Despite the name of the setting, this doesn't have to be a "no-reply" type address; however if you set it to a valid address, a lot of rubbish will get sent there, such as vacation auto-replies.

If you do wish to receive replies to automatically generated messages, you can set it equal to the  address, e.g.