SMTP server & Bounced mail, why?

SMTP server & Bounced mail, why?

I have a dedicated server (ip: 88.198.12.72) that i use as mail server
(SMTP+IMAP). The host name is mail.acreativefarm.com. Everything seems to
be right, i correctly set the MX dns entry, the reverse DNS host name,
also a SPF dns record. I tested dns settings using debounce.com and the
test is ok. Then i tried
http://www.dnsstuff.com/tools#dnsReport|type=domain&&value=acreativefarm.com
and i didnt find any critical error. But most of the times when i send an
email it returns as blocked by the destination mail server. I'm going
crazy... what i'm doing wrong? Thanks in advance, Stefano.