A number of failed emails were found in the email log: https://app.infusionbusinesssoftware.com/settings/organisation/emails/failed?where=emailState%3DFAILED.
Discovered that the email logo was ‘broken’ in Email Settings. https://app.infusionbusinesssoftware.com/settings/organisation/email
The email logo was re-loaded and all failed emails were able to send.
It is not known how the logo became ‘broken’.