CAUSE
Exchange Server 5.5 has an option to require credentials when connecting to the Internet Mail Service. Exchange 2000 also provides this option with SMTP.

RESOLUTION
Use one of the three options below.

NOTE: Do not combine the first option with either the second or third option.
• Click to select the My server requires authentication check box, which is located in the Outgoing Mail Server properties in Outlook Express.
• On the Exchange Server 5.5 computer, open the Internet Mail Service properties. On the Connections tab, click Accept Connections. Click to clear the Only from hosts using Authentication and the Clients can only submit if homed on this server check boxes.
• On the Exchange 2000 server, enable Anonymous Access on the Authentication tab of the SMTP virtual server.

Important After you enable anonymous access, Post Office Protocol version 3 (POP3) clients cannot send.