Ok, guess we are in the same båt, so to speak.
FYI, the 6.0.1 upgrade did not fix this issue but their support promised us: "Support for the HTC Magic and onwards which use activesync software by HTC is only planned from ZCS 6.0.2."
... let's hope 6.0.1 is not too long-lived
While I'm waiting I've setup a second mail profile configured as IMAP (against the same zimbra acc) -- then I just have to remember changing outgoing profile when replying/sending mail. Not pretty, but works..