Mail declined by recipient: Syntax error - line too long

[LinusGeffarth]

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?