Dim _frmKunden As New FrmKunden With {.DSKunden = DSKunden}
    _frmKunden.ShowDialog()

Public Property DSKundendaten As DSKunden
    'DSKunden ist über den Designer (in der Form) angelegt
    Get
       Return DSKunden
    End Get
    Set(value As DSKunden)
        DSKunden = value
    End Set
    End Property

For Each Kunde As DSKunden.DTKundenRow In DSKunden.DTKunden.Rows
   If Kunde.EmailAdresse2 <> "" Then
      'Tu etwas
   Else
      'Tu was anderes
Next

If Not Kunde.IsEmailAdresse2Null() AndAlso Kunde.EmailAdresse2 <> "" Then
    outZeile += Kunde.EmailAdresse2 & Feldtrenner
Else
    outZeile += Feldtrenner
End If