Just in case others are as forgetful as me ...

As Zoho has changed the POP server for personal and free organisational users, I needed to change the POP server on my email client. This failed persistently but eventually I remembered that I had chosen to use an application-specific password with Zoho, not my account password, for email access.

I set this up some time ago and had forgotten. I needed to reset the application-specific password and then I could change the settings in my email client.