Description:
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I'm not sure whether this is a sendmail or a PHP bug, but since PHP 5 my scripts fail where I use a 5'th argument like
"-f George Bush <george@oil.com>" (pardon my politics :-) ) where "oil.com" would be my domain. 
I get errors now that Bush@oil.com is not a valid username.
In PHP 4.2.3 this used to work.

When I use "-f george@oil.com", everything is ok.

I do not use safe mode, nor any settings in php.ini.

Reproduce code:
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mail( "me@where_ever.org", "Vote me!", "said it...",
        "From: George Bush <george@oil.com>\r\n",
        "-f George Bush <george@oil.com>" );


Expected result:
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A mail with in the header:
Return-path: George Bush <george@oil.com>

Actual result:
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In my sendmail log:

Bush@oil.com is an unknown user.