This is a very common phishing scam.  If you  re-validate your mailbox according to their instructions, they will use your mail server to blast out millions of spam.  Until your hosting/email account gets blacklisted and shut down.

 

Subject:  Email Alert

Your mailbox has exceeded it storage limit please reduce your mailbox size. Clickhere <http://some-strange-address.com/> . To re-validate your mailbox.
Thanks.
System Administrator.