system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is it that I receive spam emails trying to get me to open a file, but if I try and send a reply to the address, it gets rejected?
gpt
When you have control of a mail relay (or find an unsecure one), you can pretty much list whatever you want as the source address. Sometimes spammers intentionally use fake addresses, and many times they don't know how to configure their own software so they leave it as the default. Looking at the headers can reveal the actual return path. 

That said, **do not reply to those messages**. Don't even open them. Just delete them immediately.