Mail declined by recipient: Syntax error - line too long

I cannot send emails to *some* email addresses. They are rejected with the following message:

> Syntax error - line too long

and some more info from here:

>Action: failed
Status: 5.0.0
Remote-MTA: DNS; mx-ha02.web.de [212.227.17.8]
Diagnostic-Code: SMTP; 501 Syntax error - line too long
Last-Attempt-Date: 
Final-Log-ID: i04d66w419XirMM

Any idea what I need to do?