Received first spam mail to a unique email address only used for plex...

Hi,
I was wondering if anyone knows, how someone could get the email address, that I only use for plex.
(I have a unique email alias for every internet service that I use to block the ones, that deliver spam.)
It is not saved on my Mac’s Adress Book (only locally saved in an encrypted keychain).

As the title already says, I received spam to my plex email address.
This is what it said:

I am not posting this here to point fingers here, just wanted to let you know. I’d be interested if others might have received similar mails or have a theory about it.

ah, ok. Now as I read it, I think I can remember changing my password and that I received a warning about it back then by plex.
But it was so long ago, that I did not remember it today. Will also change my mail address then.