Actually this is not a PL/SQL Developer issue since it does work fine in other applications like notepad or textpad.
I can make it work fine in outlook as long as I am using a Plain Text or RTF format. If you have the format set to HTML it is the HTML parser that is ripping the leading spaces/tabs out, because HTML doesn't allow these things.
A work around for this in Outlook would be to change to plain text and then paste in the SQL you copied from PL/SQL Developer and then change it back to HTML and it will remain formatted correctly.
However I do not see how PL/SQL Developer could making changes to how they place the code in the clipboard to handle HTML and/or Plain Text at the same time.