I've got a ISPconfig 3 production server set up, running Ubuntu Server 9.04. My e-mail gets delivered ok to almost every other server I send mail to except for one (smtp.chello.nl which bounces my email). In my /var/log/mail.err I found the below error.

Sep 23 08:59:33 <MYHOSTNAME> postfix/smtp[26944]: 3DB2B1456149: to=<<RECIPIENT>@chello.nl>, relay=smtp.chello.nl[213.46.255.2]:25, delay=2, delays=0.02/0.01/1.9/0.04, dsn=5.1.0, status=bounced (host smtp.chello.nl[213.46.255.2] said: 550 5.1.0 Dynamic/Generic hostnames are blocked. Please contact your Email Provider. Your IP was <MY IP>. Your hostname was ??. (in reply to MAIL FROM command))

What could be the cause of this?

I did an SMTP check on mxtools.com and got the following:

OK - Not an open relay
OK - 0 seconds - Good on Connection time
OK - 1.482 seconds - Good on Transaction time
OK - 83.161.xx.xx resolves to a83-161-xx-xx.xxx.xxx.nl
WARNING - Reverse DNS does not match SMTP Banner

Update:
My IP is static.