The following was taken from Mercury v1.31 Mguide Help File:
Disk space restrictions: Mercury now handles disk quota restrictions on mail directories in a graceful manner. If a message exceeds a user's directory quota, it is staged for up to 72 hours before being failed and returned to its sender. In particular, Mercury 1.31 handles the boundary condition, where a message takes a user over his restriction limit, much better than previous versions, which could occasionally handle it badly.
In the past, we have not recommended the use of space restrictions at the directory level when using Mercury: this is no longer the case - we now suggest directory restrictions as the preferred way of restricting mailbox size when using Mercury, as opposed to the use of volume restrictions."
I would recommend sysadmins upgrade to the latest version or stop using disk quotas on their servers