You can save emails to your computer by saving them to a .eml format. This is done by selecting the message in Mailbird and open the additional options by clicking on the small downward pointing triangle in the message's header on the right.
Then click on 'Message source'. Now save this as a file somewhere on your computer.
Another option is saving email as PDF. To do this, click "Print" on selected email and then choose "Print to PDF".
Is there something else I can help you with?