title: Flask mail security is not meeting Microsoft Outlook's security requirements?
body: <p>We have a web application that sends emails to clients and the web application is using Flask mail framework to handle that. About 2 weeks ago, our web application failed to send emails to clients and our own group of people. We use Office 365's Outlook as our sender.</p>
  
  <blockquote>
    <p>Remote Server returned '554 5.6.0 Corrupt message content; STOREDRV.Deliver.Exception:ConversionFailedException; Failed to process message due to a permanent exception with message Content conversion: Corrupt summary TNEF content. ConversionFailedException: Content conversion: Corrupt summary TNEF content. [Stage: PromoteCreateReplay]' 
    Original message headers: </p>
  </blockquote>
  
  <p>This is the error message that the sender receives after being instructed to send email out. We contacted our Office 365 admin and Microsoft told him that the security that our web application has does not meet Microsoft's requirement / protocol. </p>
  
  <p>The question is is Flask mail using the older security protocol or configuration that is not working well with Microsoft Outlook? </p>