POP mail failing -ERR Service Unavailable

POP mail failing -ERR Service Unavailable

At least two users on the domain proptech.aero are experiencing the following error:

Task Receiving reported error (0x800CCC92) : 'Your e-mail server rejected your login. Verify your user name and password for this account in Account Settings. The server responded: -ERR Service Unavailable

The user names and passwords are correct as we can successfully log in to zoho webmail. 

Please can you advise?