Thank you for all who helped me.
I did other tests, then it turned out that I can receive e-mails from Hotmail!
So it happens that I cannot receive e-mails from Gmail, and yesterday from http://www.ruby-forum.com, because until this morning I still do not receive their e-mail for my registration.
So at least I can receive from Hotmail, but not from Gmail nor http://www.ruby-forum.com.
It is up to you guys whether to consider this case as "solved" or "unsolved".
Regards,
Bill