Those folders are showing all of the emails in them on Zoho.com and in the Zoho Mail app on my iPhone and my iPad.  They also all show up fine in Mail.app on my Mac.

The problem is that certain (not all) folders show as empty in Mail.app on iOS (iPhone and iPad).  I have tried removing my Zoho account from my iPhone and iPad, and I have tried wiping my iPhone and iPad, and setting them up from scratch.  The same folders still show as empty.

Just to be clear, in the Mail.app settings on iPhone and iPad I have set "Mail Days to Sync" to "No Limit", so it's not that inhibiting the display of the affected emails.

Any help gratefully received!

Stephen