Mail server rejecting default credentials in smtp settings for asp.net 4.0 healthMonitoring

Question

I have successfully set up health monitoring for logging errors on my ASP.NET web page to the Windows Event Log, a SQL Server database, and through email (Microsoft Exchange) when I specify a user name and password in the web.config file. However, if I change from specifying a user name and password to defaultCredentials="true" in web.config, I get the following error message in my Windows Event Log when it tries to generate the email:

System.Web.HttpException (0x80004005): Unable to send out an e-mail to the SMTP
server. Please ensure that the server specified in the <smtpMail> section is
valid. ---> System.Net.Mail.SmtpException: Mailbox unavailable. The server
response was: 5.7.1 Client does not have permissions to send as this sender

I am running Windows Vista on a corporate domain. My Windows login is identical to my Microsoft Exchange login. Can anyone provide some insight as to why specifying my login credentials explicitly in the web.config file works, but using defaultCredentials="true" does not? Are there any known solutions so that I can have an automated email sent through healthMonitoring without having to store my user name and password in the web.config file?

Answer 1

Since I earned the tumbleweed badge for this question, I doubt an answer will be of much value to anyone else; but knowing that I will inevitably fall into the same trap at a later date, I thought I would post an answer to my own question...

Authentication is not necessary for sending emails within the same domain; so instead of specifying defaultCredentials="true", I removed all fields related to authentication, and the emails began working again.

Note that this is only a partial solution. I only need to send emails to addresses within the same domain for now. Sending emails outside of this domain will not work without authentication, so if/when that is needed, it will be back to the drawing board...