In non-LMTP environments, the MTA queue directories in the Message Store system are also heavily used. LMTP works such that inbound messages are not put in MTA queues but directly inserted into the store. This message insertion lessens the overall I/O requirements of the Message Store machines and greatly reduces use of the MTA queue directory on Message Store machines. If the system is standalone or uses the local MTA for Webmail sends, significant I/O can still result on this directory for outbound mail traffic. In a two-tiered environment using LMTP, this directory will be lightly used, if at all. In prior releases of Messaging Server, on large systems this directory set needs to be on its own stripe or volume.
MTA queue directories should usually be on their own file systems, separate from the message files in the Message Store. The Message Store has a mechanism to stop delivery and appending of messages if the disk space drops below a defined threshold. However, if both the log and queue directories are on the same file system and keep growing, you will run out of disk space and the Message Store will stop working.