$ host mail.nadir.org
mail.nadir.org has address 217.114.68.227

$ telnet 217.114.68.227 25
Trying 217.114.68.227...
Connected to 217.114.68.227.
Escape character is '^]'.
220 mail.nadir.org ESMTP
helo localhost
250 mail.nadir.org
mail from: foo@nadir.org
250 ok
rcpt to: foo@so36.net
553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)