Some e-mail client programs, such as Microsoft Outlook Express, let you send large e-mail messages in several fragments. By default, when FPE scans fragmented messages (content type: message/partial), the e-mail message may be treated as a FragmentedMessage. In this case, the message is replaced with deletion text.
You can prevent FPE from deleting fragmented e-mail messages by disabling the Delete partial SMTP messages setting.To prevent the deletion of fragmented e-mail messages
In the Forefront Protection 2010 for Exchange Server Administrator Console, click Policy Management, and under Global Settings, click Advanced Options.
In the Global Settings - Advanced Options pane, under the Deletion Criteria section, clear the Delete partial SMTP messages check box, and then click Save.