Prevent Disk Space From Vanishing
Single-instance storage makes a small message addressed to multiple recipients take up more space than you might expect.
by Ben Schorr and Jim McBee
June 2003 Issue
For This Solution: Exchange-aware anti-virus software
Q A couple of our users ran out of mailbox space on our Exchange server recently. They were surprised because they didn't think they had that many items. I discovered that a small newsletter our marketing department sends out was consuming more than 350K in their mailboxes. The newsletter itself is only about 5K. Do you have any idea how this could have happened? Is it a bug in Exchange?
—Betty, Kansas City, Mo.
A Ben: We'd need a little more information to detect the problem correctly, but I'll go out on a limb and guess that the marketing department sent this item out as a blind carbon copy (BCC) to a large number of users. The newsletter itself might be only 5K, but message sizes can grow dramatically when you add a large recipient list.
Jim: That's a pretty good guess, and it's not especially intuitive. You'd think that each BCCed message would contain only the address of its specific recipient—but remember that Exchange has single-instance storage. Messages sent to multiple users aren't doled out individually; one copy is placed in the store, and each user gets a pointer to this message, which is 350K in this case because of all the recipient information.
Ben: What you can't see can fill your mailbox. The next question is what to do about it. I can think of two pretty good suggestions.
Jim: Spam alert! Tell the marketing department that nobody reads its newsletters anyway, so they might as well stop sending them.
Ben: Not a bad idea, but not where I was going with this. My first thought is to have marketing set up a mail-enabled distribution group (or distribution list in Exchange 5.5) for all the addresses, instead of BCCing them individually. This should reduce the message's size. Just keep your group sizes less than about 5,000 recipients until you can deploy Active Directory 2003.
Jim: Another solution is to use public folders. Create a public folder for marketing and have it post its newsletters there. Users must remember to check the public folder, but then the newsletters don't count against their mailbox quotas.
Ben: As long as we're on the subject, I'll toss something to you Exchange 5.5 administrators out there. Exchange 5.5 has some problems displaying the size of very large mailboxes accurately. I saw a site once where the private store was growing rapidly and the server was running out of disk space, but the mailbox resources list showed no unusually large mailboxes. It turned out that an automated help-desk application was pumping messages into a mailbox that had grown well over 1 GB. The mailbox resources screen showed the mailbox size to be quite small, however. The giveaway clue was that the mailbox had more than 50,000 items in it, even though it was reported to be less than 10K.
Jim: That's a good argument for limiting not only mailbox size, but also the maximum number of recipients a single user can send messages to.
Ben: And for keeping an eye on your automated programs.
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