not receiving mails from 1 domain . error bounced 554 5.6.0 Invalid content

Hello,

We are sellers of Astaro. One of our customers cannot receive mails from this domain walla.com

i added this domain into sender exceptions but it didn't help!!

This is the error :

2012-10-15 08:35:55 idob@alut.org.il P=<ed_ba1@walla.com> R=static_route_hostlist T=static_smtp: SMTP error from remote mail server after end of data: host 198.28.123.247 [198.28.123.247]: 554 5.6.0 Invalid content

What can i do so that astaro will not see it as spam and bounce emails back!

Thanks