By default, Microsoft Exchange does not support plain text authentication methods for security reasons. This leaves us with the following options:
A. Authenticate over a connection protected using TLS/SSL
You need Rebex Secure Mail to be able to use TLS/SSL. For more information about this, check out the TLS/SSL tutorial.
B. Use NTLM authentication
To utilize NTLM authentication, call the Smtp, Imap or Pop3 object's Login method with an optional argument which specifies that NTLM is to be used. Unlike plaintext authentication, NTLM is allowed over an unencrypted channel by default.
C. Enable unencrypted plain text authentication
If none of the two options above is suitable for you, the only solution is to enable plain text authentication for non-TLS/SSL connections using Exchange Management Console.