Adding a mail message to a mailbox is different from updating an existing mail message. Here is a slightly different description of the same principle. There are permissions which regulate access to resources. Some users have read only permissions and some may add resources or overwrite them.
Some users have the permission to place an exclusive lock on the resource.
Same conclusion as before. All other access can be regulated by the permissions. In conclusion I don't even see the need for shared locks. I have a feeling we're talking past eachother here. In fact, I'm not at all sure who I agree with and who I disagree with because I'm not clear on who is using what terminology.