IMAP no longer collecting mail from 20th June

IMAP no longer collecting mail from 20th June

I'm receiving mail OK in the Zoho webmail client.  However starting on 20th June received email is no longer downloading to my external mail client via IMAP.

It seems to connect OK, but it appears to think there's no new mail to collect.  IMAP is enabled in my Zoho mail settings.  I've tried toggling IMAP on and off, but no joy.

Not sure what to try next.  Any ideas?