E-mail (POP) not working

[Paulo Henrique Baptista de Oliveira <baptista@linuxsolutions.com.br>]

    Hi all,
    I have working in a ISP that POP stopped to work for the clients
with modem. Altought I can telnet port 110 the clients with modem
receive the following error

Error: Your server closed the connection. This could be problem with
server, network or large inativity period.

    I have installed exim and qpopper (both running in inetd).
    What else I can test?