Custom Validation for Email

I am creating a system in visual web developer. I want to use the custom validation to validate an email address.
I am using Visual basic to do it in.
so far i have
Dim ampposition As Integer
        Dim dotposition As Integer
        For i = 1 To Len(txtemail.Text)

        Next
As i want to use a for loop to validate the email
I am now stuck on this could anyone please help me solve my problem


Thanks

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