In case others are having the same problem that Alessandro is experiencing
here is a forward of a message that Gareth sent from the MPC awhile back.
I sent this to my ISP and they fixed the problem.

From: "Gareth V. Williams" <graff@...
To: mpml@...
Subject: {MPML} Accessing MPCORB

Following a number of enquiries on this mailing list and via e-mail, I
shall summarise the security checks used to determine whether access to
computers here at the CfA is permitted.

1) E-mail

Two checks are made with incoming e-mail:

a) Does the e-mail originate from a site on the spammer blacklist?
If so, bounce the message.

b) Do a reverse DNS check on the site from which the e-mail
originates.
Is it properly DNS registered? If not, bounce message.

2) http

No checks are made since Web access is only browsing.

3) anon-ftp

Do a reverse DNS on the requesting site. If not properly
registered, refuse connection.


The problems a number of you have been reporting are almost
certainly due to the fact that your ISP is not properly registering its
machines.
Note that is not sufficient that the outgoing request contain either
the machine's IP address or host name (or both): the machines here
must be able to quiz a DNS host to convert the address to the name (or vice
versa). It is up to the ISP to properly ensure that all their hosts
are properly registered.

These checks are security measures to try and ensure that all
communications can be tracked back to their (valid) source. This is
especially
important with the recent increase in attacks on high-profile sites. You'd be
surprised how often hacking attempts are made on the machines on the Harvard
network (including one very successful attempt on some CfA U**x machines a
year
or two back). But, then again, maybe you wouldn't :-)


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Gareth V. Williams, MS 18, 60 Garden Street, Cambridge, MA 02138,
U.S.A.
Associate Director, IAU Minor Planet Center
E-mail address suppressed
http://cfa-www.harvard.edu/iau/mpc.html
OpenVMS & RISC OS: refined choices in operating systems