My email isn't syncing across my decives

My email isn't syncing across my decives

Hi,
I've been using zoho mail on my iphone for a few years now.  It runs through apples mail app.
I've got it running through 3 devices and recently discovered that it's not syncing across them all.
I mainly use it on my iphone (main device) and have recently configured it on my new Macbook Air and there are now over 800+ email in my inbox and it wont sync to the content on y iphone.
I aslo logged into the main zoho mail via chrome and there are now 800+ emails there too....
Any suggestions about how I can solve this?

Cheers
Jamie