You can go through the following steps:
> long press on account name > Options and folders > scroll down to the bottom of the list and enable the option "Show other folders.."
Now you should see "Other folders" in the account folder list for this account:
> tap on this item and choose the (prior) defined "Archive" folder.
> Open the "Archiv' folder, search your 'important' message(s) and move it (them) back to the Inbox (or to any other folder in this account) by using the action icon "Move to folder.."
