I just had that error as well. In my case, one of my calendar resources had gotten it's mailstore set to the wrong server. So it was trying to send the delete command to the MTA.
To see if this is what is happening to you, run this command:
That will tell you where it thinks that conference room address lives. Now, take a look at the other calendar resources. Are they the same or different? Is the server listed a valid mailstore? If its wrong, you will need to reset it to a real mailstore before you can delete it.
zmprov ga | grep MailTransport
In my case, it was like this:
After it is moved you should be able to take care of it. I'd make sure to check all of your resources. In my case, I was not able to send messages to it.
zmprov ma firstname.lastname@example.org zimbraMailTransport lmtp:my.mail.server.com:7025