Setting mail queue filesystem free space

You can specify a minimum number of free blocks on the queue filesystem using the MinFreeBlocks (b) option. If there are fewer than the indicated number of blocks free on the filesystem on which the queue is mounted, the SMTP server will reject mail with the 452 error code. This invites the SMTP client to try again later.

Beware of setting this option too high; it can cause rejection of email when that mail would be processed without difficulty.