Use apenas uma sessão de telnet para testar a entrega de e-mails. Como exemplo,
# telnet host.domain 25
Trying host.domain...
Connected to host.domain.
Escape character is '^]'.
220 ESMTP
HELO example.com
250
MAIL FROM:<[email protected]>
250 ok
RCPT TO:<[email protected]>
250 ok
DATA
To: [email protected]
From: [email protected]
Subject: a test message
Test message body.
.
250 ok