Set store.dbtmpdir equal to /tmp/msg-<instance>. This parameter is related to Store DB Cache Size. Make sure the /tmp/ partition (or another tmpfs based file system) has enough free space to hold the database cache, that is the value of the store.dbcachesize.
Note -- the msg-&60;instance> is important, do not use just /tmp/, a subdirectory must be used. The file names placed in this location conflict with the MTA parameters of IMTA_TMP and IMTA_SCRATCH.