ho controllato nei log: /var/log/mail.log e ho trovato "Connections will not be accepted from (IP address), because the ip is in Spamhaus's list;".
ho controllato con http://mxtoolbox.com/SuperTool.aspx e risulto nella blacklist Spamhaus PBL.
Nel loro sito c'è scritto che "THE PBL IS NOT A BLACKLIST. You are not blacklisted for spamming or for anything you have done. The PBL is simply a list of ALL of the world's end-user broadband IP space, i.e: IP space normally assigned to broadband/ADSL customers. It is perfectly normal for dynamic IP addresses (DSL, DHCP, cable, dialup) to be listed on the PBL. In fact all IP addresses in the world which are not designated mail server machines *should be* on the PBL.

The PBL does not prevent you sending email unless your email program is not authenticating properly when it connects to your ISP or company's mail server. This can happen if you have forgotten to turn on 'Authentication' in your 'outgoing mail' account settings, or if the username/password your email program is giving to your outgoing mail server is wrong.

If you are using a normal email program, such as Outlook, Entourage, Thunderbird, Apple Mail, and you are being blocked by a Spamhaus PBL listing when you try to send email, the reason is simply that YOU NEED TO TURN ON 'SMTP AUTHENTICATION' in your email program's account settings."
come faccio ad abilitare in locale questa autenticazione?