Option Explicit On
 
Imports System.Net.Mail
 
 
 
 
Public Class Form1
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
 
        Dim message As New MailMessage
        Dim mycredentials As New System.Net.NetworkCredential
 
        mycredentials.UserName = "mustername"
        mycredentials.Password = "somePW"
 
        message.IsBodyHtml = False
 
        Dim mysmtpsrv As New SmtpClient()
 
        mysmtpsrv.Host = "smtp.seite.de"
        mysmtpsrv.Port = 25
 
        mysmtpsrv.UseDefaultCredentials = False
        mysmtpsrv.Credentials = mycredentials
 
 
 
        Try
            Dim strBox1 As String
            Dim strBox2 As String
            Dim strBox3 As String
 
            strBox1 = TextBox1.Text
            strBox2 = TextBox2.Text
            strBox3 = TextBox3.Text
 
            message.From = New MailAddress("account1231@web.de")
            message.To.Add(strBox1)
            'If TextBox1.Text = "" Then
            '  MsgBox("Bitte Empänger eingeben!", MsgBoxStyle.Information, 
            ' '"Information")
            '   Return
            'End If
            message.Subject = strBox2
 
            'If TextBox2.Text = "" Then
            '  MsgBox("Bitte Betreff angeben!", MsgBoxStyle.Information, 
            ' '"Information")
            '   Return
            'End If
            message.Body = strBox3
 
            'If TextBox3.Text = "" Then
            'MsgBox("Du kannst keine Nachricht mit leerem Inhalt versenden!", 
            ' 'MsgBoxStyle.Information, "Information")
            'Return
            'End If
            mysmtpsrv.Send(message) //Da ist der Fehler
 
            MsgBox("E - Mail erfolgreich gesendet", MsgBoxStyle.Information, _
              '"Information")
 
        Catch ex As Exception
            MsgBox(Err.Number & ex.Message & ex.StackTrace.ToString, _
              MsgBoxStyle.Information, "Information")
        End Try
    End Sub
End Class