I noted this reply by Patrick in another forum (thanks Patrick)
I tried it and it appears to work
"The solution for me was to select a different Outgoing Mail Server. My Exchange account was set to use the "Exchange SMTP" server (which worked fine for me prior to Mavericks). The other option in the pull down menu was something called "Exchange (Exchange)" so I picked it. This solved the problem. Sent mail once again appears in my Exchange Sent Mail folder."