how to migration exchange'mail to dovecot?

[576709166]

hi :
   i want to migration all users's mails of exchange server to our dovecot:but  
i don't know how to do this ?
   i know that exchange server can put users's mails to file.pst  , but i don't 
know hwo to convent file.pst to file.eml .
               please help     thanks !