PHP mail() issue

Posting here as I thought someone here might have seen this behavior…

When sending mail via PHP's mail() function, ~10% of my user's receive the email malformed, consistantly (i.e. not a random issue with mail() but perhaps a configuration issue with mail that is causing certain mail providers to display it incorrectly?)

Here is what these user's are seeing in the body of the email for some reason:

Reply-To: "Test User" <test@domain.com>MIME-Version: 1.0

Content-Type: text/plain; charset=UTF-8
Message-Id: <20100374043720.7BFBC2D7DB@mx.example.com>
Date: Tue, 18 May 2010 00:37:20 -0400 (EDT)</test@domain.com> 

And here are the header settings I am passing to mail():

'From: "Webmaster" <webmaster@example.com>\r\nReply-To: "Test User" <test@domain.com>\r\nMIME-Version: 1.0\r\nContent-Type: text/plain; charset=UTF-8'</test@domain.com></webmaster@example.com> 

Any ideas? Again, most of my user's see the message ok, so I am stumped. Anything I am obviously doing wrong here in the settings, or has anyone encountered this issue before?

Thanks!