This is the secondary account "send" stamp for the message.
This property applies to a MAPI message object. For a received message, the secondary account "send" stamp indicates which account a forward or a reply should be sent with, if the forward or reply cannot be sent with the primary account. For an outgoing message, the secondary account "send" stamp determines with which account to send the message, if the message cannot be sent with the primary account. Its value is the PROP_ACCT_SEND_STAMP value from the IOlkAccount interface of the account with which the message is being sent.