If my smtp on my Exchange server 2003 receive a mail with more than one in the from field (firstname.lastname@example.org; email@example.com) it fails with the following message:
220 EXCH01.cargodom.local Microsoft ESMTP MAIL Service, Version: 6.0.3790.3959 ready at Fri, 3 Sep 2010 08:56:48 +0200
250 EXCH01.cargodom.local Hello [10.10.20.109]
mail from: firstname.lastname@example.org; email@example.com
501 5.5.4 Invalid Address
The same happens sometimes when I receive mails with multiple recipient.
The recipient in the from or to field are separated with semicolon.