No Stupid Questions

It is trivial to write a piece of software, or use existing email software, to forge the contents of the from: field in an email header. In fact, you can forge the entire email header if you feel like, and there's really nothing stopping anyone from doing it. The header information which includes the alleged sender of the email is just plain text. You can fire off any email containing any header -- forged or not -- at any mail server and the data will at least get there. What the mail server does with it afterwards is up to however it's configured.

There are various techniques that email providers and mail relays use to attempt to verify the integrity of email messages, including DKIM, reverse DNS or PTR record, and the Sender Policy Framework, and if any of these don't check out the mail server may reject incoming messages or automatically divert them to spam folders. This isn't foolproof, though, and some mail servers are more lenient than others. Many private mail servers are also misconfigured, or minimally configured, and allow pretty much any damn fool thing to get through.

When you send an email to a mail server, you can set the "FROM" address to literally anything. The mail server does not care and forwards stuff on, as long as you're authenticated. Anyone can run their own mail server anywhere that will dutifully just relay emails, which is what spammers often do. There are entries in DNS called SPF records (Sender Policy Framework) which mailservers use to validate on the receiving side that the FROM address coming from the mail server matches with a list of allowed mail servers IP address(es). If it doesn't match it gets sent to spam, or outright rejected (depending on if the record says ~all or -all). It is often not ideal to reject any message that fails this check, because if you have some local system that runs its own mailserver and sends alert emails it might not necessarily match.

Look up the instructions for your mail provider to set up SPF, DKIM, and DMARC records in your DNS so email services know which emails sent from your domain are actually legit. Without those records telling email servers what's valid and how to handle what's not, it's basically the Spiderman pointing at Spiderman meme.

https://www.namecheap.com/support/knowledgebase/article.aspx/317/2237/how-do-i-add-txtspfdkimdmarc-records-for-my-domain/

The trick is setting up your own email server that has some basic common security features disabled. That way they can send email from any address they want.

I would suggest setting up DMARC, SPF, DKIM. I have it setup so mail servers would reject/drop e-mails if they do not originate from specific mail servers. If a spammer tried to send an e-mail like you describe then it would be rejected by my mail server or mail server provider.

information: https://www.cloudflare.com/learning/email-security/dmarc-dkim-spf/

There are many ways they could be getting your email, but first, why your email? Well, it's because it the easiest way for them to fake legitimate messages. Now, where are they getting them from? Most likely your socials or other accounts. One instance I've experienced was on offerup (don't use it, too many scammers).

Is it possible they actually accessed your account? Alternatively, do you have a lowercase “L” in your name that they could have replaced with a capital “I”?