Email config error

Hi, I’m getting an email that looks like this when I do a dummy affiliate registration. Obviously I don’t want my affiliates to see an email like that. I want to configure so that they can recognize where it’s coming from. I checked the documentation and found a “useful plugin” you suggested by Callum McDonald, but it has been almost 12 months since it’s been updated and I’m running WP v.3.9 now. Any suggestions on how to fix this? Thanks!!