Outlook Desktop ignores or inactivates From address in programmatically generated files

I’m programmatically generating emails using Python within a Flask application. These emails are intended to be sent from a shared mailbox via the desktop Outlook application.

In the code, the From: field is explicitly set to the shared email address (e.g. email address removed for privacy reasons). The application generates and downloads an .eml file, which is then opened in Outlook for review and manual sending.

However, when the .eml file is opened in Outlook, one of two undesired behaviors occurs:

1. The From field automatically switches to my individual default user email, ignoring the shared email specified in the code
2. The From field visually displays the shared email, but Outlook treats it as not actually selected / not connected, and the email cannot be sent.

In this case, manually selecting a different From address and then re-selecting the shared email allows the message to send successfully.

In other words, even when the shared email appears in the From field, Outlook does not seem to treat it as an active or authenticated selection unless it is manually toggled.

Has anyone encountered this behavior and have suggestions on how to rectify? Thank you!