Alternate email address on appleid.apple.com: No verification mail received.

When trying to verify ("Verify now") my Gmail-address as an alternate email address on appleid.apple.com, I never receive any verification email. (I have checked spelling of the email address, looked in my spam folder, sent from multiple ISPs, countries, continents.) This is my main Email-account, and it does not seem to be broken into or hijacked. The most likely reason seems to be that Apple doesn't send any verification email, presumably because the address is blacklisted (or taken over inside the Apple domain) by somebody else, which is worrisome. Anybody have a diagnosis or, better yet, solution?

Fritz