Yes, as defined in mailapi/imap/jobs.js, _accessFolderForMutation is defined to acquire a write mutex on the folder.  Anything else that wanted to mess with the message has to also acquire such a mutex and wait in line to perform its mutation.  In our kill-mutexes bug we want to change to more of a compare-and-swap style of operation for stuff like this.  In this case, since the per-account job/operation queue is serialized and all draft manipulations happen as job/operations, manipulations to local drafts are strictly ordered.  This may be slightly more complicated when we upload drafts to the server.