Our trade customers send a lot of their orders by email. Schedules from site managers, photographs of a handwritten list, drawings for a staircase, a spreadsheet of plots and quantities. They all go to one orders address, a shared mailbox in Microsoft 365 that our central sales team of fourteen works through from seven in the morning.
At about three on a Friday in June the mailbox stopped accepting mail. Anyone who sent an order after that received an automatic reply from Microsoft saying the recipient's mailbox was full. Nobody at our end saw anything, because a full mailbox does not look full from the inside. The team carried on clearing what was already there, went home at five, and came in on Monday to an unusually quiet inbox.
By then about two hundred and thirty orders had bounced. Some customers rang their branch. Some sent the order again on Monday. A groundworker who needed blocks on site at seven on Monday bought them from a competitor on Saturday morning, and told his account manager so.
A shared mailbox without a licence of its own has a limit of fifty gigabytes. Ours had passed forty nine about five weeks earlier, which is when the warnings started. They were sent where Exchange always sends them, to the mailbox that is filling up. In a personal mailbox that works, because the owner reads it. In ours, each warning arrived among six hundred customer emails a day, was opened by whichever of fourteen people reached it first, and was left for IT, who everyone assumed already knew. Nobody in IT received a thing.
We used to watch disk space on our own mail servers as a matter of routine. When mail moved to Microsoft we stopped, as if the capacity had moved with it. The limits had only moved somewhere we were not looking.
That Monday we gave the mailbox a licence that raised its limit to a hundred gigabytes and added an archive, and set a rule to move anything older than a year into it. Then we looked at the rest. Six other shared mailboxes were above eighty percent of their limit, two of them in finance. Mailbox sizes are now collected every night with everything else we monitor, and anything above seventy five percent raises a ticket for my team rather than a message to the mailbox. We also send a test email to each of our critical addresses every hour and alert if it bounces.
A warning that goes to the thing in trouble is only useful if somebody owns that thing and reads it. Send capacity alerts to the people who can act on them, and remember the limits that did not leave with your servers.
– Serguey Shinder
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