iPhone Sent Folder cannot be changed

Hello!

For quite a while now, I have been unable to specify an IMAP folder as the Sent folder on my iPhone.

My email account is an IMAP one through a minor ISP. In that account, I have a folder that is used by Mail.app on my Mac at home. Let's call it "Sent-IMAP".

On my iPhone, the default is a folder called "Sent", and is not stored on the IMAP server. It appears only on the iPhone.

I have tried to change the folder by doing this:

1. Settings
2. Mail, Contacts, Calendars
3. Under Account, Tap the IMAP account
4. Under IMAP, Tap "Account (email@isp)"
5. At the bottom, tap "Advanced"
6. Under Mailbox Behaviors, tap, "Sent Mailbox"
7. Under "On the Server", tap the "Sent-IMAP" folder. A blue checkmark appears on that line
8. Tap "< Advanced" at the top
9. (It now shows, under Sent Mailbox", "Sent-IMAP", where before it showed "Sent")
10. Tap "< Account" at the top
11. Tap "Done"

However, it does not stick. If I go back through that process (steps 1-5), it shows the Sent Mailbox as "Sent" again, not "Sent-IMAP"

Does anyone have any ideas as to why I can't specify the IMAP folder? My Mac can!