Possible causes: 1) General access denied, sender access denied - the sender of the message does not have the permissions ...

{4}: Possible causes: 1) General access denied, sender access denied - the sender of the message does not have the permissions required to complete delivery. 2) You're trying to relay your mail through another SMTP server and it doesn't permit you to relay. 3) The recipient might have mailbox delivery restrictions enabled. For example, a recipient's mailbox delivery restriction was set to receive from a distribution group only and non-members' email will be rejected with this error. 4) For Exchange Server 2003, a distribution group can be configured to restrict mail delivery from unauthenticated users. Mail that's sent by using an unauthenticated SMTP session is rejected.