newbie question

[Dario Rossi]

Hello all.
Sorry for boring you, but i have this problem i cannot understand:
I configured qmail with tcpserver following the installation instructions
step by step.
At this point i have a tcpserver at the moment allowing
connections only from localhost.
Everything is working fine and i send and receive e-mails with no
problems.
If i try to do relaying from a host that is not allowed i get the message:
"553 sorry, that domain isn't in my list of allowed rcpthosts".
The question is :If i try to send an e-mail to a domain that's contained
in rcpthosts, from an unautorized host, i get no errors and the email
arrives...why this?
with this method can't i be able to mail bomb any e-mail address of this
domain?

Thanks 
Dario