I have an issue where a user connected to his corporate mailbox via a MacBook using OWA (I don't have any version details). He composed a message with three recipients. The message was successfully RECEIVED by all the recipients, but the SENDER was someone else - i.e. another valid user account in the company was listed as the sender. Message tracking from the person composing the message does not list the message in Sent Items. The message does show up in the Sent Items folder of the second account (i.e. the account that shows up as the sender in the recipient's mailboxes). Neither of the two "sending" mailboxes have full access or send as permissions for anyone other than typical server/service accounts and administrators. I've never seen this issue before.