I received a message from Dropbox about my account that addressed me as 'Random' (not my real name). I have no account with Dropbox. I use this pseudonym on a few fora (I did a search and see there are a lot more other users of RandomTroll who aren't me.) but not for any other purpose (it's an inside joke from my college days, >40 years ago).

It would seem that either one of those fora gave out my address or someone hacked them. Has anyone else seen this?

If you have things registered online, expect spam & act accordingly.

My brother registered a site online and told me once about the spam he would receive, including an email offering him a mailing list (for him to send spam to) including the owner of every registered domain, which of course included his email. I posted regularly on a few mailing lists when running an Electronics business and acquired a similar problem, because email addresses were harvested.

If I had a nickel for every piece of spam I've gotten about my account at some bank I have never dealt with, I could afford that new Mustang. Spammers send stuff out at random.

Once your email address gets out there even once, spammers trade it back and forth. It's highly unlikely that anyone entrusted with it gave it out knowingly, but don't rule out stupid. Also, if your address was at some site that got hacked, it would similarly be fair game. For example.

Do a web search for "email addresses hacked." I think you will find the results interesting.

I wonder if it's a Yahoo account? Over a Billion "served"
https://haveibeenpwned.com/