title: sending e-mail via exchange unsuccessfully
body: <p>I can't find anywhere where I have a problem sending emails, it only happens to some users, while for others everything normally works with this code:</p>
  <pre><code> Dim service As New ExchangeService(requestedServerVersion:=ExchangeVersion.Exchange2013_SP1)
      Sub SendMail(ByVal Mailadressen As List(Of String), ByVal CCAdressen As List(Of String), ByVal Betreff As String, ByVal MailText As String, ByVal Anhang As List(Of String), Optional ByVal Absender As String = &quot;&quot;)
          
          service.Url = New Uri(&quot;https://mail.server.com/ews/exchange.asmx&quot;)
        
  
  
          'Add a valid user credentials  
        
          service.Credentials = New WebCredentials(&quot;someuser&quot;, &quot;somepasw&quot;, &quot;somedomain&quot;)
         
  
          'To address the SSL challenge 
          ServicePointManager.ServerCertificateValidationCallback = New RemoteCertificateValidationCallback(AddressOf ValidateCertificate)
  
          Try            'Create new message object and set properties required to send a mail 
  
              Dim sender As New Microsoft.Exchange.WebServices.Data.EmailAddress
              Dim message As EmailMessage = New EmailMessage(service)
              message.Subject = Betreff
              message.Body = MailText
  
              If Absender &lt;&gt; &quot;&quot; Then
                  Absender = Absender
                  sender.Address = Absender
                  message.From = sender
              End If
  
              For Each adr As String In Mailadressen
                  message.ToRecipients.Add(adr)
              Next
  
  
              For Each cc As String In CCAdressen
                  message.CcRecipients.Add(cc)
              Next
  
              For Each a As String In Anhang
                  message.Attachments.AddFileAttachment(a)
              Next
  
             
              message.SendAndSaveCopy()
            
          Catch ex As Exception
              MessageBox.Show(vbNewLine &amp; &quot;&quot; &amp; vbNewLine &amp; String.Format(&quot;Error: {0}&quot;, ex.StackTrace))
          End Try
  
      End Sub
  </code></pre>
  <p>error I'm getting:</p>
  <blockquote>
  <p>At Microsoft.Excange.WebServices.Data.ServiceRequestBase.GetEwsHttpWebResponse
  (IEwsHttpWebRequest request) at
  Microsoft.Excange.WebServices.Data.ServiceRequestBase.ValidateAndEmitRequest
  (IEwsHttpWebRequest &amp; request
  .WebServices.Data.ExchangeService.InternalCreateItems (Inumerable1
  items, FolderId parentFolderId, NUllable1 messageDisposition,
  NUllable1 sendInvitationsMode, ServiceErrorHandling errorHandling) at
  Microsoft.Excange.WebServices.Data.Item.InternalCreate1
  messageDisposition1 .WebServices.Data.EmailMessage.InternalSend
  (FolderId parentFolderId, MessageDisposition messageDispostition)</p>
  </blockquote>
  <p>can you please give me some suggestion what could be the problem here? thanks a lot</p>