Hello Sadeq,
Just to give you more insight on SMTP authentication here is a small excerpt from wikipedia.
From Wikipedia
Differently from mail-access protocols, the original SMTP specified by Jon Postel in the 1970s did not provide for using passwords for sending email messages. This lack of security gave rise to open mail relays, unprotected mail servers used to propagate spam and worms that became a plague in the late '90s.[2] Before SMTP AUTH, a relay client had to be identified by IP address, which is only practical for email services provided by the same Internet service provider (ISP) supplying the connection, or else using specific hacks, such as POP before SMTP.
John Gardiner Myers published the first draft of SMTP AUTH in 1995, and it has been successively developed and discussed in the IETF along with mail submission protocol, Extended SMTP (ESMTP), and Simple Authentication and Security Layer (SASL). An older SASL mechanism for ESMTP authentication (ESMTPA) is CRAM-MD5, and uses of the MD5 algorithm in HMACs (hash-based message authentication codes) are still considered sound.
From the error description it looks like your SMTP server does not allow it to be used as Open Relay & requires authentication before an email can be sent out. You can ask your administrator to allow sending emails from the IP address of the server on which your ASP.Net application, but it's not really a good idea. Instead check your web.config and in that check the mailsettings element. It should something like one shown below.
<mailSettings>
<smtp deliveryMethod="network" from="ben@contoso.com">
<network
host="host name"
port="integer"
defaultCredentials="true|false"
userName="string"
password="string"/>
</mailSettings>
Regards,