[Flask] Security used by Flask mail causing issue with Outlook?

[Cal Wong]

Hello Flask team,

My name is Calvert and I hope this is the right place to post which I think is a bug (I could be wrong) that prevents our web application to send out emails.

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:

This is the error message that I obtained from the Outlook sender after trying to send out an email. Our Office 365 admin contacted Microsoft and Microsoft told him that our web application that uses Flask mail does not meet their security requirements which causes the issue.

Our web application works fine before but it failed to work about 2 weeks ago. We did some testing and I will list them out there.

1) We copied an application that sends emails from a Flask tutorial and we successfully sent emails out using the same email that we are using for our web application.

2) We have 3 different folders, the development, staging, and the production. The web app in the development folder runs on local host, staging and production folders are hosted on Heroku. We managed to send out emails by running the web app in the development environment but failed to do so on the staging and production environment.

3) Instead of using Outlook as our sender, we tried using gmail as our sender and our web app was able to send out emails.

4) We tried using another Outlook email as our sender and it fails to send out emails.

>From what I have listed, it sounds like our implementation of Flask mail in the web app is faulty but we did not make any changes to the code and we did not change the email content. The web app managed to send out email if Heroku is not involved. In addition with what Microsoft said, I don't know which party went wrong and causes this issue that we are having.

The developer that made this web app is no longer working with us and I will try my best to provide anything needed to solve this issue. It would be great if you can confirm that Flask mail is indeed working well with Outlook.

Thank you for your time and I appreciate all your response in the future.

Regards,
Calvert

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