I've just passed through a weird issue, when after reconfiguring Outlook Mail, the "Sent items" folder was not showing the sent e-mails. Of course, I made sure to tick the checkbox that tells to store the sent items.
After a long search, I found the most ridiculous solution from "Hawk", at:
This problem is very easy to fix if this is an IMAP mail folder.
You may notice that your drafts aren't saving either, in fact nothing is going into the folders from your computer when you produce a mail. You may find that mail produced from other devices appears, but not the mail from your windows 7 computer. You may also notice that your folders, Drafts, Junk E-mail, etc are not positioned at the left hand side of the list, but are in fact tabbed across a bit, right of the inbox. Is this so?
The Solution is simple:
1) Right Click on the account and open Properties
2) Go to the IMAP tab.
3) Enter "Inbox" into the Root folder path *AND HERE's THE TRICK: IT MUST BE "Inbox", not "inbox" the entry is case sensitive.
4) Press apply and let the folders reorganise themselves. They should now reload and all be on the left hand side.
Done. New sent items, drafts etc will now move to the correct folders.