BPROPFIND throwing bad request

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Hi,
This is the webdav query i am sending for retreiving start date and
end date for large number of appointment occurences:

<?xml version='1.0'?>
<a:propfind xmlns:a='DAV:' xmlns:b='urn:schemas:calendar:'>
<a:target>
<a:hRef>{2077A07E-0B39-4B84-AFDC-59D01CFD3441}.EML</
a:hRef>
<a:hRef>{2077A07E-0B39-4B84-AFDC-59D01CFD3441}-2.EML</
a:hRef>
</a:target>
<a:prop>
<b:dtstart/>
<b:dtend/>
</a:prop>
</a:propfind>


Follwing is the url on which we are querying:

http://<exchangeServer>/exchange/<username>/Calendar", "BPROPFIND"
.