What are mail folder subscriptions in my mail client?
A mail client will not automatically sync with all folders when connecting to your email account via IMAP. It will sync with the INBOX, Drafts, Sent, Spam, Trash, & Archive folders by default. User-created folders can also be subscribed to, but are not included by default. Additionally, a folder must contain messages in order to be selected.
In Roundcube, you can navigate to Settings » Folders to see a list of your available and subscribed folders:
If you are using Outlook, you can use the following instructions provided by Microsoft to subscribe to your IMAP folders: