Smtp-Testando o Smtp Relay do seu servidor.

abaixo um exemplo prático de como testar se o relay do seu servidor esta aberto:

myhost:~$ telnet mailserver.example.com 25

Trying 192.0.2.82...
Connected to mailserver.example.com.
Escape character is '^]'.
220 babbo.example.com ESMTP Sendmail 8.13.4/8.13.4; Tue, 20 Sep 2005 16:31:04 -0300
helo myhost.example.net
250 babbo.example.org Hello IDENT:1008@myhost.example.net [192.0.2.44], pleased to meet you
mail from:
250 2.1.0 ... Sender ok
rcpt to:
550 5.7.1 ... Relaying denied. Proper authentication required.
quit
221 2.0.0 babbo.example.com closing connection
Connection closed by foreign host.

aparecendo
Relaying denied esta tudo ok.