Copio ed incollo il testo del seguente articolo che spiega di problemi di invio posta da OWA di un Exchange 2003 ad un Exchange 2010 tramite Owa. La soluzione proposta è l'installazione di un hotfix oppure la disabilitazione del messaggio di Disclaimer:
http://howexchangeworks.com/2012/05/exchange-2010-transport-rule-disclaimer-co-existence-owa-2003.html
I came across an issue where the email message sent from OWA 2003
using IE8/9 (while co-existing with Exchange 2010) delivers a lot of
garbage as the message body. This only happens if you have a 2010
transport rule for disclaimer.
The below screenshot shows the type of email you get as a recipient.
It looks like it is a known issue and the KB article
explains the issue. The problem occurs because Outlook Web Access for
Exchange 2003 sets an email to an incorrect character set. When the
message is routed by the Exchange 2010 server that has the disclaimer
rule enabled, the email is converted to incorrect HTML tagging based on
the incorrect character set. Then, the message is corrupted by the
incorrect tagging.
The fix is to install a hotfix on Exchange 2003 servers in the
environment and a registry edit to enable the hotfix. The server needs a
restart as well.