promotional emails from mailer-daemon@icloud.com

They're not; iCloud has nothing to do with it. Some scammers have figured out how to use mailer-daemon@icloud.com to make their emails look legitimate. I just got swamped with them, too; the first one I got was about my supposed lack of iCloud storage. I have to admit it LOOKED convincing, and might have made me pause for a second if I were, in fact, actually running out of storage space — but the copy was a joke. It was yelling about how my account "might" be at risk because I "might" be running out of space because my storage "might" be full because they can't "process my account storage." Bravo, scammers! Then came more emails from "Endurance" "Renewal by Andersen" "Compression Socks", etc., all from mailer-daemon@icloud.com. They're so painfully laughable. So I was curious to see if others here were suddenly experiencing it too.

Since legitimate emails from mailer-daemon@icloud.com are just notifications about undeliverable emails, I'm going to go ahead and block the address for now. Being notified if an email I sent wasn't delivered isn't as important to me at the moment as stopping this flood of crap (entertaining though it may be). :-)

mailer-daemon is the standard address for bounce back messages that are sent to an unknown address. If you are receiving these and they are not from emails you have sent, then there are 2 possible causes.

• Spoofed sender address - It is common for spammers to spoof the sending address and those are not actually coming from iCloud. On your Mac you can view the Header of the email and the last hop on the bottom of the header is the one closest to the actual sender. The fields such as To and From are just text strings that you see in Mail and are routinely altered in the header by the spammer to make blocking ineffective. Apple would like you to report these emails with this recommendation: If you receive a suspicious email that looks like it's supposed to be from Apple, please forward it to reportphishing@apple.com
• Account compromised - This is not likely, but if someone does have access to your account, they could be using that email address to send their ads to others and would be bounced back to your iCloud address. I say this is not likely because a bounced back email would say that it was not deliverable and would not appear to be an advertisement simply emailed to you. For a compromised account, the solution would be to change your Apple Account password, but I do not suspect that to be the problem in your case.