OK... this is solved.
It was me
AntiSpam/AntiVirus (recommended):
ClamAV, Amavis & Spamassassin, Requires 2Gb+ RAM
rDNS Check (recommended):
Drop all emails if no rDNS/PTR
Install DKIM & SPF (recommended):
Installs DKIM & SPF, enables DKIM for New Accounts and Domains
Install Policyd (recommended):
Installs Policyd, enables hourly email limit per domain. I needed to go in to the Postfix mail manager and tick the following boxes:
Once the mail server was rebuilt, the inbox mails started to flow back in to the mail clients.
Such a stupid thing not to have done... Sometimes I can't see the wood for the trees