You are using Symantec Premium Antispam with Language Identification enabled and the subject of some messages are appearing as question marks when the message is viewed in Outlook.
When the feature "Language Identification" is enabled, SMSMSE notifies Premium Antispam that this feature is enabled while submitting any mail message for spam scanning. After scanning of such a message, Premium Antispam returns the verdict "is- known-language" and then SMSMSE adds tracker strings in the message header. Because of these tracker strings and along with the Premium Antispam verdict "is- known-language", the subject gets altered and hence the question marks appear.
The Language identification feature is used in conjunction with the Symantec Outlook Plug-In. The Symantec Outlook Plug-In is only currently supported with Exchange 2000. Most organizations will not be using the Outlook Plug-In.
Disable language identification feature: