Do not use hardcoded folder names for iCloud Mail accounts
iCloud Mail allows users to choose which folders to use for sent- and deleted messages. The default IMAP folder names are “Sent Messages” and “Deleted Messages”. However, for compatibility with Outlook desktop, I have changed them to “Sent Items” and “Deleted Itesm”, and deleted the original “[...] Messages” folders. In this scenario, the Outlook mobile app won’t be able to perform message deletion, because it uses hardcoded names for special IMAP folders.
Please refer to support ticket 9913608 for details.