Outlook Desktop ignores or inactivates From address in programmatically generated files

The user is creating emails in a Flask application using Python, designed to be sent from a shared mailbox via Outlook. The application generates an .eml file that specifies the shared email address in the "From" field. Upon opening the .eml file in Outlook, the "From" field either reverts to the user's individual email address or displays the shared email but renders it inactive. Outlook seems to not recognize or authenticate the provided shared email as a valid sender address. The core issue is that Outlook does not allow the email to be sent when the shared email is selected until the user manually reselects it. This suggests a problem with how the .emk file is being created or how Outlook handles the shared mailbox credentials. The user seeks a solution to prevent this manual intervention and ensure the .eml file correctly sets and activates the shared email in Outlook. The problem likely stems from authentication or authorization issues within Outlook due to the way the shared mailbox is configured. Ultimately, the goal is to have the .eml file open in Outlook with the shared email ready for immediate sending. The user is looking for advice and solutions related to how to properly configure the .emk file, or interact with Outlook to address this.