Postfix + postfixadmin = SMTP errors...

[Rashef]

I installed Postfix with Postfixadmin some weeks ago. I've been able to solve most of issues I met (several errors from receiving servers) but there is something I cannot understand yet.

This is what I get from a receiving SMTP server:

Jun 23 09:57:43 node1 postfix/smtp[3950]: send attr reason = host 
mail.domain.it[abc.abc.abc.abc] refused to talk to me: 550 
Jun 23 09:57:43 node1 postfix/smtp[3950]: 418E098004C2: 
to=<xxxxx...@mediofactoring.it>, relay=mail.domain.it[abc.abc.abc.abc]: 
25, delay=80211, delays=80211/0.03/0.45/0, dsn=4.0.0, status=deferred 
(host mail.domain.it[abc.abc.abc.abc] refused to talk to me: 550 )

I also tried:

# telnet mail.domain.it 25 
Trying abc.abc.abc.abc... 
Connected to mail.domain.it (abc.abc.abc.abc). 
Escape character is '^]'. 
550 
Connection closed by foreign host.

What's that? A sort of blacklist?

UPDATE
I checked my IP at http://mxtoolbox.com/blacklists.aspx but it seems it's not blacklisted at all... :-(