One problem that often comes up with new internet users is getting replies to emails that you never sent. Usually, you, seem to have been sending abuse or marketing emails to random people and you know you didn’t do it, but now you are getting a complaint back about it. This phenomena is more common than people realise easy to understand once the mechanics are explained, which I will do now with an analogy.
When you post a letter, you will often write a return address on the back. People can write back to this address or the post office can return the letter if it is not delivered. You already do the same thing with email, but it happens automatically because you set it up that way when you configured your email the first time.
When you post the letter you could also, in theory, write someone else’s return address on the back; the post office wouldn’t know or care and neither would the recipient. You can do this with an email too, very easily. Don’t actually do this now, in case you can’t undo it, but you would go to the tools menu and then account setting and edit the name and email address that are displayed on outgoing emails. There is usually no more validation than there would be by the post office.
This practice is used by scammers who want to conceal their identity. It is easy to spot once you learn about it and you should treat it the way you do other spam.