On logging in to Gmail they warned me my alternative Hotmail recovery email address (in case of lost password etc.) was 'not safe' and I should change it immediately.

When I logged in to check, the Hotmail a/c was indeed still there, but totally empty - not a single message left in any folder. (there is separate thread on this)

But the big question is: how did Google know something was wrong with my Hotmail?

Scary.

Just guessing but maybe they received Spam from that account into another Gmail users mailbox and that user marked it as Spam. ?

And I doubt one Email would trigger that but more then one ?

And in this case it would be very easy for Gmail to determine that your account is involved.

Edit 1: Or another guess, Hotmail detected that account is sending spam, and would inform other email providers (Spamhaus project maybe http://www.spamhaus.org/) of the account ?

Additional info.

If you have regained access of your Hotmail account, ASAP enable 2-Step Authentication

Read more about it here:

http://windows.microsoft.com/en-us/windows/two-step-verification-faq

http://windows.microsoft.com/en-us/windows/app-passwords-two-step-verification

What is 2-Step Authentication / Verification ?

http://en.wikipedia.org/wiki/Two-step_verification

They make money with spying your interests and sell it for promotions.

Both are well known to happily cooperate with any government which wants to spy on you.

Be happy that for once they used it to your benefit.

Seems it could be a scam to grt ur password, maybe any email/password would work, if i was you id go to thr actual hotmail.com site login and change your password asap

somebody now has a copy of all your emails.

any passwords, usernames or any other personal infomation is compromised.

As always, should you or any of your IM force be caught or killed, the Secretary will disavow any knowledge of your actions. Good luck, Jim. This tape will self-destruct in five seconds.