When connecting to your server’s POP3 service, your client might provide this error just after authentication:
The connection to the server was interrupted.
Your best bet is to check the mail log and see exactly what the problem is:
web pop3-login: Login: john [192.168.0.5] pop3(john): Invalid mbox file /var/spool/mail/john: No such file or directory pop3(john): Failed to create storage with data: mbox:/var/spool/mail/john dovecot: child 29864 (pop3) returned error 89
In this case, the mbox file has become corrupt (possible from malformed ‘From’ headers). You have the option of repairing the issues within the file, or you can simply create a new mail spool for the user.