PR_PRIMARY_SEND_ACCT [Outlook 2003 API Reference]
This is the primary account "send" stamp for a message.
This property applies to a MAPI message object. For a received message, the primary account "send" stamp indicates which account a forward or a reply should be sent with. For an outgoing message, it determines which account to send the message with. Its value is the PROP_ACCT_SEND_STAMP value from the IOlkAccount interface of the account with which the message is being sent.