One question we often get asked when talking to customers contemplating an Exchange upgrade or a switch from a competitor’s mail system is, "how many users per server can Exchange handle?" Nowadays, that’s an open question - it very much depends on what kind of users you have, what kind of storage you’re using and how powerful your servers are. When Exchange 4.0 was released in 1996, a decent server might have had 256Mb of RAM and a 90MHz Pentium processor, with maybe a handful of GBs of SCSI disk in the box and possibly a DAT tape drive. Users’ mailboxes might have been in the 10-20Mb size range, and the average user sent or received only a small amount of email per day.
At that time, server sizing was pretty much a function of how much computing horsepower you could afford - the CPU power, disk size & speed and memory capacity available (along with the all-important user profile) would determine the number of users per server, and arbitrary decisions would be made about maximum message sizes, mailbox size etc. Now, it’s possible to buy even mid-range servers that will cope with many thousands of users, and the bottleneck has moved down to the storage level in many instances as user mailboxes have grown in size and we send and receive far more mail, and many larger messages too.
We’d be interested in hearing anecdotes about particularly unusual messaging environments you might have - how large do your servers grow? What’s the biggest mailbox you’ve ever seen (assuming you’re not restricting with quotas)? How much data does your total Exchange system manage? What’s the biggest single database - etc, etc... you get the picture.
Some factoids to get you started...  if you have anything along these lines, please post a comment:
- One customer (who is part of the Exchange Technology Adoption Program) sets no limit on message sizes internally - and one time saw a single message with attachments totaling 2.4Gb in size! (which Exchange delivered! - the recipient’s mailbox quota was instantly blown apart though...)
- The largest single Exchange 5.5 Public Folder Information Store I’ve come across was 800Gb in size
- There’s a limit of 1,000 Exchange 2000/2003 servers in an org - has anyone come close, or hit that limit?
- One customer deploys around 250,000 Exchange users per server and has over 9,000,000 users in total (you guessed it, they’re an ISP)
- The most heavy user of public folders in the world (that we know about) has about 1,500,000 public folders containing around 8Tb of data
So come on: let’s hear your war stories - post a comment with your amazing data :)
- Ewan Dalton

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After reading some of the factoids my story has little weight. We have three users with mailboxes above 10 GB, the largest of them being 15 GB.
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One unusual thing we do with our Exchange setup is we run a 365 day deleted item retention time. This is because we are a Performing Arts Hall and our business is molded around a performance season, which is around a year.

Our Exchange system is NOT large though, not by the standards you stated above, but one thing I can say is that about how the deleted items affect the store size. When we migrated from E2000 to E2003 we moved the mailboxes from one server to another new server. Before moving the mailboxes my Information Store was pushing 16GB (I also upgraded to Enterprise...) but AFTER moving the mailboxes, the Information Store was just 4GB! Since deleted items do not migrate, I had almost 12GB of deleted items! Crazy huh?
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15Gb mailbox? I think it's time for an intervention.
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Well maybe not that extreme but we have a single server environment (Exch2k3) with about 1500 mailboxes which contains:
- 1,2Tb mailstore
- average mailbox 500Mb
- Maximum mailbox 7-8Gb (there are several)
- largest database is 310Gb
Server is performing good but storage (SAN) is a real bottleneck, disks are spinning at maximum rpm. Furthermore:
- no quota's
- 365 Deleted item retention
- since users started mailing ISO's there is a 20Mb send/receive limit.
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#2Getting that call at 2am Friday morning "We ran DS/IS of our 250GB pub 2 hours ago, when will it complete replicating the folders from the 100+ site from around the world?"...answer=two weeks.
#1Everyone is afraid to goto the CEO of the huge company desk and figure out why his Outlook takes 20min to open. His mailbox is 7.5GB on the server, 150000+ items... Still had to goto his desk... 124k in the Inbox 37 items... deleted items 7.4GB 149943 items. And you make 100x my salary?

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Largest current mailbox store: 213GB
Largest mailbox (total items): 398k
Largest mailbox (size): 24.3GB

No management support for any restrictions on mailboxes. These are attorneys and their time is too valuable to waste any on cleanup.
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A great post on the Exchange Team Blog about some of the more strenuous situations that Exchange has...
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Ewan has written a thought provoking post on the Exchange team blog.  He talks about organisations...
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Hi there, love the Blog

Here we have approx 20,000 users most of our servers are 2x PIII @ 1GHZ with 4GB or RAM (HP DL380 G1 - will be upgradeing to IBM blades soon) connected to a EMC Clarrion CX600. The total size of all of our DB's is about 3.5TB Each server has about 760GB or data/server and is has 4 storage groups with 5 mailbox stores per db.Each SG is on its own RAID 5 lun with it own TL (Raid 1) volume. Each server has about 2-3000 IO/S of storage connected to it.

Company wide we have about 14 Exchange servers:

5x SAN connected mailbox servers
2x Bridgehead+ PF servers
2x Front-end servers
4x Remote Exchange servers running local Public stores & connectors to our main hub site

Bottlenecks on our end:
Our processors are working a little to hard & we'll be moving to 3GHz Xeon processor IBM blades soon. During business hours each of our servers averages 200-600 IO/S with temp spikes of 1000+
I'd say our biggest Bottlenect is 32-Bit windows. We've been fighting Paged Pool Exhaustion issues on EX2K & EX2K3. If Exchange supported 64-Bit addressing we could load up our servers with oodles of Ram for a Large DB cache which would hopefully provide better performance.

Largest Mailbox ~12GB
Our corporate office has ~500 users but consumes about 25% of our storage.
Avage mailbox size (company wide) 250MB

Oh well, that my 2 bits... looking forward to see what everyone else is running.

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I am the admin for an SMB with about 50 users. I am in the process of helping some of our more "abusive" users cleaning up their mailboxes which range up to over 1GB in size. It is slightly hard to justify moving to the enterprise edition for such a small group of users and that 16GB limit is very liiting for today's standards of email usage.
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With such large storage requirements, I'd like to know how everyone's backup strategy looks like. When do you have time to do online defrags or how much of it is needed? What is your typical recovery strategy if one of you large databases get corrupt. How long does it take to recover the database etc.

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We're much smaller than many others here... but just thought i'd include ours since we seem a little odd - given what we use.

SBS 2000 with approx 40 users.
priv info store is 13.8gb

largest mailboxes: about 5 with >1gb, maybe one or two with >2gb

retention: 15 or 30 day

GPO: outlook ask to delete items in recycle bin

obviously we have no quotas... and additionally we have a digital scanning device sending through email (approx 100 docs @ 80kbps/ea daily) which i've got set to auto delete after some time...

with our anti-spam throwing emails into recycle automatically, the GPO really helps people stay on top of deleting junk.

our backup is currently veritas to a tape drive, though i'm trying to look at using our point-to-point T1 to do some sort of sync (to a NAS) or VSS type solution. (somehow i'd like to combine VSS's history with block level sync'ing to the NAS... the backup would be off site, up to date, and have historic versioning :))

I realize we should upgrade to 2003, but our SIS ratio has historically been way to high to consider... the second we'd do that (aside from in-place upgrade or the unsupported forklift method) our info store would break the 16gb limit... hopefully w/ SP2 we can upgrade though! :)

my $.02
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one of our postmaster mailboxes (we have one for each of our 9 sites) was 23 gigs when I started here a year ago, no one had logged onto it in several years and it had over 6 million items in it. It took me 3 days to use exmerge to shrink it down. Our biggest user mailbox is about 3 gigs, with quite a few other mailboxes over 1 gig. Other than that we're pretty average (we have 8,000 users and 15,000 mailboxes). Our backups have improved dramatically at sites that upgraded their tape drives to SDLT320. At our largest site we backup 625 gigs a night (exchange mailstores only), it took 16 hours before we upgraded to SDLT320's and re-arranged our backup jobs, now it takes less than 5 hours. We're planning on buying exchange dedicated SAN's soon so we'll be moving to online snap backups of exchange, then streaming it off to tape, which will help a lot.
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not proud of those limits but Ewan, I can confirm we have the following:

- a few pub.edb over 1.4TB and still growing
- average mailbox size around 2GB
- no end-user mailbox limits
- priv.edb got close to 1TB, now broken down over many servers
- we gave an award to a user who got his mailbox moving successfully from e5.5 to e2k3 with just under 30GB...

and of course we got to replicate all this to DR sites with no data loss and 15min RTO!

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did you tell me about hardware working on this configurations?????????/??

and may be you can give me an advice.... =))
i have 200-300 mailboxes aprox 500 MB each..
and 2-3 mailboxes more than 2-3GB..

in advance thank you very much
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Largest mailbox I have ever seen was during a GroupWise to Exchange 2003 migration for a large law firm in NYC. I believe this person was a managing partner. It took me more than a week to migrate this mailbox, which topped over 35GB when done.
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If you're interested in hardware/software scalability take a look at the numbers that the MS Exchange...
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I often get to hear some “interesting” stories about sizes of peoples mailboxes but nothing...
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(This is a follow on to the previous post on measuring business impact , and the first post on the business