Checking E-mail

[jackly94]

I am writing a program that will ask the user to add in some of his details, which are stored into a text file, how do i check to see if the email he entered is not already stored in the text file already?

I am also having problems with the Error checking it trying up error when i click save

Imports System.IO
Public Class SecondForm
    Private Sub btnExit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExit.Click

        'closing program

        Me.Close()
    End Sub

    Private Sub btnClear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClear.Click

        'clearing Text boxes

        txtFirst.Clear()
        txtLast.Clear()
        txtAddress.Clear()
        txtAddress2.Clear()
        txtCard.Clear()
        txtMail.Clear()
        txtphone.Clear()
        txtPassword.Clear()
    End Sub

    Private Sub btnSave_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSave.Click

        'Declare an object variable

        Dim Accountfile As StreamWriter

        'This procedure reads data about Account from a file.

        Dim strFirst As String
        Dim strLast As String
        Dim intTelephone As Integer
        Dim strMail As String
        Dim strAddress As String
        Dim strAddress2 As String
        Dim intCredit As Integer
        Dim strPassword As String
        Dim iIn As Integer

        'Getting the File name off the User

        strFirst = txtFirst.Text
        strLast = txtLast.Text
        strMail = txtMail.Text
        strAddress = txtAddress.Text
        strAddress2 = txtAddress2.Text
        intTelephone = txtphone.Text
        intCredit = txtCard.Text
        strPassword = txtPassword.Text

        'Open the Data File

        Accountfile = File.AppendText("Account.txt")

        'Write to the file

        Accountfile.WriteLine(strFirst)
        Accountfile.WriteLine(strLast)
        Accountfile.WriteLine(strMail)
        Accountfile.WriteLine(strAddress)
        Accountfile.WriteLine(strAddress2)
        Accountfile.WriteLine(intTelephone)
        Accountfile.WriteLine(intCredit)
        Accountfile.WriteLine(strPassword)

        'Error Checking on if Number is Entered 

        If IsNumeric(txtCard.Text) Then
            iIn = CInt(txtCard.Text)
            If iIn < 14 Then
                MsgBox("Credit Card Number Not Vaild, Please Enter 14 digit number")
            Else
                MsgBox(iIn)
            End If
        Else
            MsgBox("Not a Number")
        End If

        If IsNumeric(txtphone.Text) Then
            iIn = CInt(txtphone.Text)
            If iIn < 0 Then
                MsgBox("Number not Vaild")
            Else
                MsgBox(iIn)
            End If
        Else
            MsgBox("Not a Number")
        End If



        'Close the Account File

        Accountfile.Close()




    End Sub
End Class