If you put a mail in a folder, then you can not filter the destination folder. You have to know your folder structure exactly and first click on the main folder, then the subfolder, etc. etc. to finally tap the destination folder, so that the mail is stored.
Is only a small thing, but extremely important for usability.
A supplement from me: there already exists an Enhancement Request, but I get no feedback if the ER is in progress or anything. Furthermore I cannot access the blog app :-(