Internal server error, malformed header

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When I create a new account I get sometime an error that display in the browser : 

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.

In the log there is : 

malformed header from script. Bad header="test test" <[email protected]: /www/htdocs/test/cgi-bin/php
sendmail: setgroups: Input/output error

I am running PHP 4.3.1 thought.

I seems something is wrong in my server configuration,

any idea where I should first look at ?