To manage the folders go to Accounts | Tap your Account | Folder Mapping.

To change the folder mapping, just tap the folder you wish to map, and select the folder you want to map to.

In general, it should fit the folder your provider’s web client is putting emails into.
This will guarantee your sync is correct: emails you archive/delete/send in Blue Mail will be well placed for other clients to access, and emails you archive/delete/send in other clients will sync properly to Blue Mail‘s Archive/Trash/Sent folders.

Note: Folder mapping is applied only to IMAP and Exchange and not for POP3.