How do I archive IMAP messages with non Gmail/iCloud account on iOS Mail.app?

This may seem like a simple question, however I don't seem to be able to work this one out for myself. I hope you guys/girls can help!

I have my IMAP email account setup on my iPhone (own domain name, DBmail IMAP mail server running on VPS). Email works great; I can send/receive, delete, move messages no problem.

When I used MobileMe, Gmail or iCloud, I was given the option to "Archive mail" in the account configuration for that mail account in Settings.app. However, with a simple IMAP account, even though I have an "Archive" folder in my IMAP mailbox, which shows up with a suitable 'Archive-like' icon in Mail.app on my iPhone, I am not given the option to "Archive mail" - i.e. move a message from 'Inbox' to 'Archive' by touching the Archive button - Instead, I just get the delete button, which just moves messages to my 'Trash' folder in my mailbox.

Up to now, I've just put this down to Mail.app on iOS lacking this functionality. After upgrading to iOS 6 this morning (which I hoped would bring this functionality) I'm starting to question whether there's a setting or a workaround that I've missed that provides this functionality. I'm not sure I understand why Mail.app on my MAC allows me to Archive mail for my simple IMAP account, but my iOS devices won't.

Does anyone know of a way to get a simple IMAP account setup in Mail.app on iOS to 'Archive' mail, as opposed to delete? Please chime in if you have any suggestions.

Joe