MAIL-Versand mit STARTTLS ? PowerShell

Send-MailMessage : The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 SMTP; Client was not authenticated to send 
anonymous mail during MAIL FROM [......prod.outlook.com]

function AVIS_Mail($Mail, $body, $Mail_File, $Subject) {
     try {

     [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12 # bringt leider nichts

     $SMTPServer             = “smtp.office365.com”
     $SMTPClient             = New-Object Net.Mail.SmtpClient($SmtpServer, 587)
     $SMTPClient.EnableSsl   = $true
     $SMTPClient.UseDefaultCredentials = $true
     $SMTPClient.Credentials = New-Object System.Net.NetworkCredential("user", "pw")  
     
    
     $Mail_Msg               = New-Object System.Net.Mail.MailMessage
     $Mail_Att               = New-Object System.Net.Mail.Attachment($Mail_File)  
     $Mail_Msg.Attachments.Add($Mail_Att)
     $Mail_Msg.from          ="Avis@.....de"   

     $bccMail                ="..@.de"  
     $Mail_Msg.To.add($Mail)
     $Mail_Msg.Bcc.Add($bccMail)
     $Mail_Msg.Subject       =$Subject
     $Mail_Msg.IsBodyHtml    =$true
     $Mail_Msg.Body          =$body

     $SMTPClient.Send($Mail_Msg) 
     } 
    
     catch 
     {
      $_.exception.message
      $Mail_Msg.Subject    = "Fehler beim Zustellen einer Nachricht"  
      $Mail_Msg.Body       = "Fehler! Es konnte keine E-Mail an: "+$Mail+" geschickt werden<br><br>$_.exception.message"  
      $Mail_Msg.To.add(".....")  
      $smtp.Send($Mail_Msg) 
      break;
     }
     #Prozess wieder freigeben, da sonst Zugriffsfehler
     $Mail_Att.Dispose()
     $Mail_Msg.Dispose()
     [gc]::Collect()
}