You can have all your WordPress email traffic come and go from a single Gmail account. Therefore you can have many sites sending emails from one address, making it easier to reply to your users. You must have at least one email that works to use Email Tunnel. That one free email can be a free account like Gmail.
First, connect your “tunnel exit” site to a free gmail account. This can be accomplished with this plugin for free: https://wordpress.org/plugins/wp-mail-smtp/
Once your tunnel exit is connected to Gmail, you can connect unlimited entrance sites with the Email Tunnel plugin. At that point, all your WordPress emails will be routed to and from Gmail, and you can use the Google web interface to manage them.