The current process to deliver attached files inside the email is not efficient for the smtp protocol and servers, neither for the user, due to file size limitation.
With SquirrelMail, as a web interface for email, it should be possible to store any uploaded file on the web server, and sending in the corresponding email a link to the file, instead of the encoded file.
So the email would be lot lighter, and the security issue could be solved through the existing https features.
Interested in any further idea...
Best regards, and thanks a lot for this already very efficient software and project.