Hola cerra, es porque no le pasas el empleado a tu consulta:
Código vb:
Ver originalPublic ssql As String
Public lectorExtensiones1 As SqlDataReader
ssql = "SELECT email FROM mail INNER JOIN empleados ON mail.codempleado = empleados.codempleado where empleados.codempleado=" & comboBox1.text
Dim cnnCargar As New SqlCommand(ssql, cnn)
cnn.Open()
Ddatareader = cnnCargar.ExecuteReader()
If Ddatareader.Read Then
txtboxemail.Text = Ddatareader.Item("email")
Else
MsgBox("Email no encontrado", MsgBoxStyle.Information, "Aviso")
'Limpiar
txtboxemail.Text = ""
End If