Description:
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In the documentation to the mail function the handling of the chars \r and \n are misleading.
The explanation  from Ben Cooke in the comments is very clear. Link to the comment: http://de.php.net/manual/en/function.mail.php#59773

It would be great to find the explanation  in the doc directly. Which would help others to find this information faster.

Here is the comment:
Note that there is a big difference between the behavior of this function on Windows systems vs. UNIX systems. On Windows it delivers directly to an SMTP server, while on a UNIX system it uses a local command to hand off to the system's own MTA. 

The upshot of all this is that on a Windows system your  message and headers must use the standard line endings \r\n as prescribed by the email specs. On a UNIX system the MTA's "sendmail" interface assumes that recieved data will use UNIX line endings and will turn any \n to \r\n, so you must supply only \n to mail() on a UNIX system to avoid the MTA hypercorrecting to \r\r\n. 

If you use plain old \n on a Windows system, some MTAs will get a little upset. qmail in particular will refuse outright to accept any message that has a lonely \n without an accompanying \r.