Thank you for the information.
Client.Connect() worked for me too. In fact I was able to browse all folders on my FastMail.fm account using the same example as shared by you in your original post. I have noticed that the root folder for FastMail.fm account is the “INBOX” and all other folders (by default or manually created) are accessed like “INBOX.Sent Items” and “INBOX.Drafts” etc.
Before executing my sample project, I have created a folder on the root by name of “aFolder” (please check the snapshot attached MailBox.png). I didn’t have any messages in that folder but when my project iterated over the folders to save messages, this folder is accessed to retrieve the messages. That means, if this folder would have messages, they would have got saved. Please check other snapshots for your reference.