Chances are there’s nothing infecting your PC/Mac. Nor has Hotmail been compromised. Nor is there likely anything at all that you can do to prevent this.
The “from” field of an email is customizable. Very easily customizable. I can craft an email right now that says it’s from “jctennis123@hotmail.com” or “jctennis123@hatesyou.com” or whatever I want.
The point is, it’s outside your control. The “from” address in email is not authenticated in any way whatsoever. SMTP is inherently based on truthfulness, trust, and the golden rule… which means it’s easy to subvert to nefarious ends.
And there’s not a thing you can do about it except to consider getting some sort of third-party authentication mechanism into the mix. But then, your recipients would need to do the same thing as well to see any benefit.
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This is a common spam ploy. People often whitelist the contacts in their address book, and so this technique helps ensure that the spam stays in your inbox instead of auto-deleting to trash or junk folders.
How’d they get your email address in the first place? From marketers, or by trolling through online forums for comments associated with email addresses, or by simply sniffing network traffic. It’s easy.
Keep that in mind when emailing in general. Your email can be viewed by anyone at almost any time whilst in transit from A to B. Most internet traffic is unencrypted and openly viewable to those that so desire.