by Wolfram Saringer (2014-03-19) last change: 2014-03-19
Have a simple conversation with your SMTP server using telnet to port 25:
--> HELO youthostname <-- 250 smtpservername --> MAIL FROM: xx@xyz.xx <-- 250 Ok --> RCPT TO: <xx@xyz.xx> (place recipient address here) <-- 554 <xx@xyz.xx>: Relay access denied