masalah email saat dial lewat lain isp

Dear Postfixer,
Server email saya berisi postfix+clamav+amavis-new+spamassasin(thanks pak asfiani, ternyata pake spamassasin gampang banget).

Ber-email ria di jaringan local tidak ada masalah, tapi kalo dial pake telkomnet instan, beberapa user saya(nggak semua) tidak bisa terima maupun kirim email.
Pesen errornya kayak gini:

Your server has unexpectedly terminated the connection. Possible causes for this include server problems, network problems, or a long period of inactivity. Account: '[email protected]', Server: '192.168.0.254', Protocol: POP3, Port: 110, Secure(SSL): No, Error Number: 0x800CCC0F


Thanks & Regards,

Dadio