Step 1: Login to the e-mail account through the built-in iPhone mail client and then browse the following folders
+++
Drafts Mailbox
Sent Mailbox
Deleted Mailbox
+++
Step 2: Open Settings and then Mail, Contacts, Calendars
Step 3: Select your email account and then Select the displayed IMAP account
Step 4: Choose Advanced Settings (at the bottom) then go to Mailbox Behavior.
Step 5: You will find the following 3 options listed at the top:
+++
Drafts Mailbox
Sent Mailbox
Deleted Mailbox
+++
Step 5: These options lead to a page that allows you to configure how your phone stores a copy of drafts, sent or deleted messages.
Each page will display a directory tree available on your mail server, and you need to choose one of the folders in this directory tree (under the “On the server” section) in order to force the mail client to store the messages there.