If you started getting messages (e-mails) in root's mailbox with:
Postfix SMTP server: errors from XXX" with errors of "452 Insufficient system storage"
or getting messages in the log:
warning: not enough free space in mail queue
First thing to check is that you didn't require the queue to have minimum space of some kind, using queue_minfree in main.cf.
Hans Hofker on the debian list gave a better answer:
Apparently postfix wants the mail queue to be 1.5 times the max message size. And since you have set your message size limit to...
> message_size_limit = 7500000000
...you should have more than 11 GB of disk space available to receive a single message. (11.25GB = 7.5GB * 1.5) So you can either buy more diskspace, or reset your message_size_limit to a more realistic value.
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