Zugriff auf Mail-Relay-Server verweigert

[Architect]

Weiß jemand, was die Fehlermeldung „Relay access denied“ verursacht?

Mail-receiver relay access denied

Have this same error with mail-receiver reports: Relay access denied (in reply to RCPT TO command). Mail receiving isn’t working for new install but have got this to work before. Believe all settings are correct but might have missed something.