Which identity will be used ?

I have an intranet application in which I'm trying to send emails. For this I used the following configuration

ASP.Net : Windows authentication, IdentityImpersonate=True

IIS : Anonymous  ...... ( I know there is no need to used 'anonymous' , but I was just checking out)

I am not able to send mails with this setting. I guess this is because the application will have IUSR_MACHINE identity which is not present in LDAP server.

But guess what when I remove IdentityImpersonate=True, the Email code works.

According to my knowledge when I use 'Anonymous' with or without IdentityImpersonate=True the identity is going to be IUSR_MACHINE. Then how code the Email code doesn't work in 1st scenario and works in 2nd. Can anyone shed some light?
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