Hey there.
We have had similar problems in the past, and I feel it is starting to resurface with some profiles again.
We already fixed this bug:
Bug 48276 – MYOB Crashes when ZCO is used
and its probably related. Essentially MYOB uses a dud way to send emails through outlook, and they aren't going to fix that in a hurry. I suggest you lodge a bug and work with Zimbra on this. We have had a couple of customers report it recently.
The workaround we suggest is to install pdf creator, and print to PDF and email from there. You don't get the MYOB email address, so depending on how accurate your address book is, that can be annoying.
Happy to chat with you about it, feel free to give me a call at my office if you want to discuss it (I assume you are in Australia). Your logs reveal your server name and username btw.
Sometimes, it works by deleting the outlook profile from the mail control panel thingo, and setting it up again. That can fix it. making sure you have all the service packs for office can fix it too. or try remove the profile, remove the connector, reinstall the connector and resetup the profiles. In a terminal server this is of course messy.