I tried that a long time ago.
It only works when email sending is not part of a cronjob and when you have the correct project loaded up.
When using a cronjob for sending emails this does not work at all.
In EmailReporting we use project_override to select certain projects. Because MantisBT does not officially support using "from_email" and "return_path_email" this way, they probably did not include the usage of project_override in the sending of emails functionality.

Of course its possible to get this working but it would require changes in MantisBT to get it work reliably.

Thank you for the explanation.

We feel the sending email address is not really important for the processing to notes.
Any configured mailbox will process to any projects.
It's only an issue for the party that receives the email notifications. If that is a generic MantisBT mailbox this should not be big issue as the project is mentioned in the subject line.

If MantisBT ever adds functionality to facilitate this we will reopen this subject. For now we are closing it.