Sending an email using a fake email address is easy to do, especially for experienced hackers and spammers.
Perhaps not surprisingly, they’ve used that ability to send emails that look as if they’re from trusted sources (firstname.lastname@example.org), and even from your own address. Spammers and viruses count on your curiosity, and hope that an email from yourself will interest you enough that you’ll click their link or open their attachment.
To send an email with a fake address doesn’t require your systems to be compromised at all. This can be done as easily as you can send an envelope to someone with anyone’s return address on it; so go ahead and throw it in the trash like all the other junk mail.