Keep your mail server from blacklists.
Spamhaus is used by a lot of email providers, e.g. GMX and Yahoo, to refuse incoming emails from
IPs listed there.
You can check the Spamhaus blacklists (SBL,PBL, XBL) at http://www.spamhaus.org/query/bl?ip=IP.
Instructions for avoiding the CBL are here.
The answer to the HELO should be fully qualified domain name (e.g. "mail.linuxia.de"), with correct reverse DNS lookup.