timed out while receiving the initial server greet

A weird problem with Postfix 2.3.3 running on a 2.6.17 linux system, NOT a
Postfix problem definitely, but maybe worth mentioning here.

With at least one host (maybe others, but surely < 0.1% of the hosts I
contact) when I try to send mail I get the following message in the mail log

/var/log/maillog.3:Sep x xxxx mail postfix/smtp[5720]: 386D426F683:
to=, relay=mailbox.COMETA.IT[85.33.215.233]:25, delay=25396,
delays=24796/0.14/600/0, dsn=4.4.2, status=deferred (conversation with
mailbox.COMETA.IT[85.33.215.233] timed out while receiving the initial
server greeting)

and the message gets stuck in the queue

If I try to telnet the above host on port 25, the connection is made but the
remote mail server doesn't send its 220 message upon connection. The weird
thing is that this host, apparently running IMail 8.00, needs a couple of
'enters' with telnet at least, to wake up and send the damn 220 message.

It actually *seems* an IMail problem and not a Postfix one, since the same
happens telnetting port 110 on the same host, running an IMail POP server,
BUT if I try to telnet it from some other host, even running the same
version of linux, then there is no need to wake up the remote system with
enter, HOWEVER if try other ports not managed by Imail, it works EVERYWHERE.

I 'solved' using

echo 0 > /proc/sys/net/ipv4/tcp_window_scaling

disabling tcp_window_scaling

Another machine with the very same kernel, but a different nic and different
routing does not show this behaviour. A routing problem ? A NIC driver
problem ?

Fabio Rossetti