Postler and it's "INBOX is locked" message

I'm sure some of you have had Postler stop being able to receive messages from your email account one day or another, but have had no idea why.

Running postler in a command line, I would see the following when attempting to "receive mail":

** (postler-service:2376): DEBUG: postler-service.vala:67: Line/ input: Connection is now encrypted
** (postler-service:2376): DEBUG: postler-service.vala:67: Line/ input: Logging in...
** (postler-service:2376): DEBUG: postler-service.vala:67: Line/ input: Opening slave local...
** (postler-service:2376): DEBUG: postler-service.vala:67: Line/ error: Error: channel :remote:INBOX-:local:INBOX is locked
** (postler-service:2376): DEBUG: postler-service.vala:303: Done: 0 new messages

I couldn't figure it out at first, and the first time it happened I thought it was a transitory issue since it seemed to have been fixed when I reinstalled Postler and move the old configuration directory out of the way. Then, it happened again and I had stopped using Postler to check mail.

Just yesterday, however, when I had a process to kill, I saw a postler-mbsync process while I wasn't running Postler. I killed it (killall postler-mbsync) and then started up Postler today, and lo and behold, the inbox is no longer locked and I am able to receive mail~