For now we will work on a 16GB limit. (16GB = 16384 MB) Divide this up between 100 people, and that's 163.84MB per person.

So the formula here is

Mailbox Size x Users = Total Database Size

Unfortuntately, it's not really quite that simple. There are two other considerations. Single Instance Storage and Deleted Items Retention.

When an Exchange user sends an email to two people outside the Exchange Site, each user recieves a copy of the email. When an Exchange user sends an email to two people within the same Exchange Site, there is only one copy of the email distributed. Each user actually only recieves a reference to the email. This is what's known as Single Instance Storage.

Single Instance Storage is a measurable item. You may monitor your storage ratio in Performance Monitor using the MSExchangeIS Private and MSExchangeIS Public objects Single Instance Ratio counter. Don't be alarmed that your Single Instance Storage Ratio is not very high.

For this example, we'll assume the Single Instance Ratio is 1.578.

Deleted Items Retention, you should already have an understanding of. The value set for Deleted Item Retention, is the amount of days Exchange keeps a message after a user has deleted it. We'll assume the value of this is 30days. If the current Private Information Store size is (8.9GB), this equates to 17% of this space being taken up in Deleted Item Retention. So on your 16GB limit, this makes approx 2.75GB, you loose in Deleted Item Retention. Obviously changing the value of this from 30 days, to something less will change this value.

So the true formula is

((Mailbox size x Number of Mailboxes) / Single Item Retention Value) + Deleted Item Retention Percentage) = Size of the Private Information Store

Let's apply this to your site, and assume everyone has a 100MB mailbox!

Mailbox size = 100MB

Number of Mailboxes = 100

Single Item Retention Value = 1.578

Deleted Item Retention Percentage = 17%

((100 x 100) / 1.578 + 17%) = 7.25GB (rounded)

Now if we applied this to your current limit (Final Limit not Warnings), of 349MB the result is

((349 x 100) / 1.578 +17% = 25.5 GB (rounded)

Ovbiously you cannot support that, so, yes your limits need to be reduced.

Also, as a good rule you should allow at least another 20% for growth etc.

Public Folders are a little different, but some of the same rules still apply. Although it's a little more involved. Public Folders should have limits set to their size, because they are also bound by the same restrictions as Mailboxes. Currently, you have no limits set on your Public Folders sizes. The size restrictions on Public Folders, generally need to individially configured, so working out the formula gets a bit tricky

Assuming your Public Folder configurations are currently

Public Folder Databse Size Restriction = 16GB

Current Public Folder Database Size = 753MB

Deleted Item Retention = 180 days

Single Instance Storage = 1.578

Deleted Item Retention Percentage = 28%

Public Folder Size (Per Folder) = ???? (You will have to do each one individually, or work out an average)

((Total Public Folder Sizes) / 1.578 + 28%) = Public Folder Database Size

Total Public Folder Sizes = Total Number of Public Folders x Total of Public Folders individual limits

Mailbox Manager does not work on Public Folders, however, you can set Age Limits on Public Folders. It's quite similar, where items older than the Age Limit are deleted. But remember, when using Age Limits, these are deleted permantly. Deleted Item Retention only comes into effect, when a user deleted a folder or item. The same goes for Mailbox Manager. When this deletes items, the Deleted Item Retention does not come into play, and these are deleted permantly.