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. ".