Some sites like zorpia.com allowed you to login through your gmail, yahoo or hotmail Id. How it is possible? Probably gmail, yahoo etc are validating users for zorpia on request of zorpia. But it is not secure, zorpia is getting our passwords for gmail, yahoo etc.
I never trust on a site asking me to register with login of may mail account and "corresponding-password", no matter even if the inbox is empty.
But there are other things, I hate and still continue with them, for e.g. merging of services...
Guess you become member of a SSS service, using YYY mail account as user-id. Later SSS service is taken up by XXX mail service, a competitor to YYY. SSS however has nothing to do neither with XXX and YYY. I felt the necessity that i should have XXX mail-account to continue with SSS services... something like, "SSS is open only if you have XXX account". I can not let go my YYY mail account but then i have to have another XXX mail account, where as SSS had nothing to do with such terms, it was then said,
"SSS is free. ( blah-blah ) SSS may change these Terms and Conditions from time to time. ".