Minimum disk space required for MailMarshal to send mail

It turns out that MailMarshal has a minimum amount of disk space required (512MB) that it needs to be able to process mail. If there is not enough disk space then errors will be noticed in the receiver log files, similar to the following:

3320 00:01:26.510 Diskspace: free=0x00000000208cb000, limits=(0x0000000020000000, 0x0000000040000000), avail=1% on (D:\Program Files\Marshal\MailMarshal\Quarantine)
3320 00:01:26.510 Diskspace: free=0x00000000208cb000, limits=(0x0000000020000000, 0x0000000040000000), avail=1% on (D:\Program Files\Marshal\MailMarshal\Logging)
3320 00:01:26.510 Diskspace: free=0x00000000208cb000, limits=(0x0000000020000000, 0x0000000040000000), avail=1% on (D:\Program Files\Marshal\MailMarshal\Queues\Incoming)
3320 00:01:26.510 Low disk space left, sleeping for 29s. Found 520.79 MB free on D:\Program Files\Marshal\MailMarshal\Queues\Incoming. Minimum required: 512.00 MB. Minimum preferred: 1.00 GB.

Adding more disk will resolve the issue — another reason to be Virtual!