Currently the exchange integration does not manage sync of sent items very seamlessly. Particularly if messages are sent from a mobile phone they are not part of the sync, and the user must manually move emails to a sent items sync folder, or have a complex set of server-side and per user rules, which causes duplication of emails in the mailbox.
In general, sent items also do not associate with the correct related entity. It logs the message against the user's entity or the 'from' field, but this should be the recipient or the 'to' field.
Reference support case - 025736 for all the details.