[Source]

For some reason, and I'm sure I did it myself - I just have no idea how, the mainboxes (In, Out, Drafts, Sent, etc) in Mail have suddenly changed from being aligned to the left of the application, to being aligned on the right.

Can anybody tell me how I can put them back on the left?

Cheers.

[analogika]

Assuming you are talking about OS X 10.3 or earlier:

Hide the mailboxes drawer (click on the "Mailboxes" icon in the toolbar).
Move program window over to near right-hand edge of screen (so that there wouldn't be enough room to the right for the mailbox drawer).
Show the mailboxes drawer.

Voilà.