IMAP-Folders on the iPhone

Setting iPhone Sent/Draft/Trash folders for mail accounts

These folders (Draft, Sent, Trash) can be set from the iPhone by going to:

Settings -> Mail -> (youraccount.com) -> Advanced -> Mailbox Behaviors

This section of that page allows you to select Drafts Mailbox, Sent Mailbox or Deleted Mailbox, which in turn brings up a page that allows you to specify "On My iPhone" or select one of the folders "On the Server" (at least in the case of an IMAP account type, which is where I've done this).

Note that if you are syncing your mail account settings from your Mac, whatever you set here on the iPhone directly may be overridden by the settings from your mail account as defined in Mail.app on your Mac the next time you sync your iPhone. Your options in that case are to either disable sync of that mail account in iTunes, or better yet, leave it set to sync your mail account, ignore everything I've described above, and set up the appropriate Draft/Sent/Trash folders in Mail.app instead. The next time you sync your iPhone, it should propagate the folder settings to your phone correctly. To change these folders in Mail.app, select a subfolder under your IMAP account (for example, "Sent") and then select the menu item:

Mailbox -> Use This Mailbox For -> Sent

(or Drafts, Junk, Trash as appropriate to that folder).