Fios blocks port 25 only. All the other ports are OK. Outbound port 25 has been blocked on and off in the past, most recently blocked again effective 6/1/10. Inbound port 25 is currently open. Even if port 25 is not blocked, Verizon's dynamic IP addresses are listed in both the Spamhaus Policy Block List (PBL) and MAPS DUL. The PBL is/can be used by mail exchange servers (MX's) to reject connections from these addresses. Thus, for the purpose of running a mail exchange server port 25 is effectively blocked. However, a mail server can be run that utilizes Verizon 's authenticated SMTP just like your mail client would do.