postfix: timed out while sending message body

[Aaron Pohle]

Hi,
since fedora core 4, I have a problem with postfix. Postfix sends
small messages but great
messages (>64 KByte) are not send. The logfile /var/log/maillog says:

postfix/smtp[18886]: A741C10DACB: to=<aaron.pohle at gmail.com>,
relay=mail.gmx.de[213.165.64.20], delay=206, status=deferred
(conversation with mail.gmx.de[213.165.64.20] timed out while sending
message body)

With kontact I can send large messages directly to mail.gmx.de but
this is no solution for me.
Have somebody any idea what the problem is?

Thanks,

Aaron