You could use the CalDAV API - see https://tools.ietf.org/html/rfc4791 and https://tools.ietf.org/html/rfc6638 for the scheduling extensions

It would help to know what you are trying to achieve. Note that you cannot set any attributes using GetDistributionListRequest. To do that, you need ModifyDistributionListRequest or, if you are wanting to add members - AddDistributionListMemberRequest remove members - RemoveDistributionListMemberReq...

Perhaps worth enabling "dav" tracing on the server for the user and looking at what commands are being executed against it by the client. Enable using: addAccountLogger user@example.com zimbra.dav TRACE Disable when finished capturing data: addAccountLogger user@example.com zimbra.dav INFO

See https://tools.ietf.org/html/rfc6764#section-5 for details of the standard Zimbra is following/assuming.

Is there anything interesting in /opt/zimbra/log/mailbox.log at the time you see this?

If you can see the message in Zimbra Web Client and do a "Show Original" for it. Please submit a bug at https://bugzilla.zimbra.com/ explaining your problem. I'm guessing there is something specific to that message that our Outlook connector is having difficulty with.

I regret that Zimbra does not support attachment handling via CalDAV.

Please vote for https://bugzilla.zimbra.com/show_bug.cgi?id=84191 if you want to see that feature.

Sorry about being in a time warp.
A show original for the calendar items and perhaps the invitation messages sent out might provide a clue.
If you can watch and give a step by step recipe for creating the problem?

Are you really running version 6.0.2?
I'd recommend upgrading to a newer, supported version and see if the issue is fixed.

See https://files.zimbra.com/website/docs/8.6/Zimbra_NE_Admin_Guide_8.6.0.pdf If a COS is not explicitly set for a new account, or if the COS assigned to a user no longer exists, the Zimbra default COS is automatically assigned. You can create a domain COS and have all accounts created on that domai...