Some outright sell email addresses you give them, (like the cashier asks for one every time you get a receipt, or one of countless "giveaways" ) so if you can, use one of those "hide my email" deals. Apple offers one with iCloudPlus ($), and some other ISP's may do so. They let you use a cryptic email address that forwards to your real email address, and you can shut it down any time. (like right away, after you get that "free book" that requires an email address to receive. ... and brace yourself for endless marketing)
Another is to get a "throwaway" email address (receive only) at a place like getnada.com.
Give someone your getnada email addess, pick up your freebee there and forget it.
The addresses are open to anyone who can guess it, so nothing personal, please. Delete any messages.
And the inbox is emptied once a month.
Perhaps more likely, some place was hacked and emails harvested. Happened to me (I changed password on that email address) when there were hacks at LinkedIn, Yahoo, who knows how many others.
You can check if your email was harvested at https://haveibeenpwned.com
This is legit and has a Wikipedia entry https://en.wikipedia.org/wiki/Have_I_Been_Pwned%3F
Just make sure that you change the password on any of them since they were harvested.
Some got passwords. Definitely don't use easy ones, and change them on those accounts.
Matter of fact, I haven't seen this recently, but some say "We have your password and know all the porn you've been watching, Please contact us ... to send money" but even if you have been watching porn, these are empty threats. (and who watches that stuff? 
)
USPS can very legitimately get your email address if you sign up for Informed Delivery.
It uses email to notify you of letters and packages coming.
The others, less legitimate.
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