How many Mailboxes can I put on my server?

This is a very common question that I get asked, unfortunately there is not really an easy answer. To ensure that get the best out of your Exchange 2007 server you should have the following information to ensure you do not overload your mailbox servers.

1. The Number of Processors

2. Exchange Server Profile Analyzer

3. The amount of Memory

With this data you should be able to decide whether your existing hardware is up to the job or what hardware you will need to buy

In general 1 processor core will accomodate 1000 average users. What is an average user I hear you ask - well that is were the Exchange Server Profile Analyzer comes into play. Using this tool you can connect to an existing Exchange server and pull the data from here. If you do not have Exchange at present then you can use the following as a rule of thumb.

This is assuming a send /recieve per day of an approx 50kb message

Light User 5 sent / 20 recieved

Average 10 sent / 40 recieved

Heavy 20 sent / 80 recieved

Very Heavy 30 sent / 120 recieved

Therefore if you have 4000 average users you will need 4 Core processors.

Likewise if all your users are Heavy users then this will half you number of users per core (500 per core).

Some caveat is if the mailbox server will be running LCR as this will add an additional 20% overhead to processor utilisation and needs to be taken into account. Likewise if you are going to have multiple server roles this will reduce the maximum number of users that you can host on the server by 20% assuming you are hosting both Hub Transport and CAS.

This should allow you to work out the number of processor cores that are required so we now have to work out memory

Again user profile is a deciding factor here and depending on if your users are light/average/heavy will determine how much memory.

Light User 2Gb plus 2 Mb per mailbox

Average User 2Gb plus 3.5 Mb per mailbox

Heavy 2 Gb plus 5 Mb per mailbox

So if you are intending to have 1000 light users you would be looking at 2gb + (1000 x 2mb) which is 4gb of ram.

Again there are caveats - If running LCR you should add an additional 1gb. If you are running multiple server roles then you will need to add the additional mininum (2gb for Hub - 2Gb for CAS)

There are other considerations which I will blog later but as a high level design objective to work out either what your hardware will support or to work out what hardware you need to buy this is a good starting point

Find more here