In this article i’m giving some best suggestions that can be scoped in-terms of storage  for any new implementation and redesigning of existing environment for Exchange messaging Platform.

There is no such in recommendation of giving a standard best practice for mailbox size, Maximum message size , retention and archive  as it varies according to each every org structure based on Storage , Email flow per day , Server hardware , memory, requirement  etc..,

Before taking up any new implementation its always better to set up scope on the below things

1) Mailbox Size per user.

2) Maximum Size of Message.

3) Retention of Mailbox items.

The first step we will look in planning mailbox size for users
The better way i would suggest is to first collect the daily, weekly and monthly email statistics for the whole environment

These amazing scripts from nuno mota will help you on collecting the statistics which is very useful.

Emails sent per hour – https://gallery.technet.microsoft.com/office/ec7940c1-be9a-45d5-9ea1-921677fa081c

Emails sent per day and size – https://gallery.technet.microsoft.com/office/f2af711e-defd-476d-896e-8053aa964bc5/view/Discussions

Emails sent per week and size – https://gallery.technet.microsoft.com/office/Total-Emails-Sent-and-c1daf5e7

Calculate the average size of the emails sent per user on daily basis from the above output

Based on that calculate the mailbox quota that you need to set for all the users .
You need to make sure that all the mailboxes are balanced equally on existing mailbox servers before planning.
Measure the currently allocated disk size in each mailbox server based on the current value and expected value in 1 year.
multiply the value and calculate the growth of expected users in 1 year.
Calculate the current log space , database space and the expected value after 6 months
Always keep a buffer value of 30 percent on the total value of the disk space required to be on the safer side.

Better you can set some exclusions for mailbox quota for  VIP users.When we say VIP users you can better define a policy with the client before taking up the project. Example for executive users the mailbox size will be unlimited and for the number of users and fix a static value for the remaining mailboxes and finalize the agreement. Define these policies clearly before taking up any new project.

Because a poor capacity planning will bring the following causes :

1) Increased storage cost as days passes.
2) More network bandwidth usage between servers and clients since all the users will have their emails stored in their mailbox.
3) Affecting the storage subsystem in terms of I/Ops.
2) Maximum message size

Planning for maximum message size for the whole organization is also very important.

Following things can be impacted of improper message size planning

1) First thing will affect you is mass mailing from the organizations itself (HR,Marketing Team). They will start sending mass mails with large attachments if the message size is not restricted properly.Impact of this will be your hub servers will go down.
Always you need to give a clear note of this part during the planning and make a best practice of sending mass mails after production hours since the normal emails will be affected.

2) It increases the network utilization of messaging system which will create again an impact.

3) If the network is compromised by a hacker then he can utilize the network/system resources by sending emails with large attachments.

4) Also it will be difficult for anti-spam solutions to scan these emails with large attachments and more possibly exclude most of the attachments if they are encrypted with large size and no exclusions set for encrypted attachments in the anti-spam filter.

3) Retention and archive

Planning for retention and archive is very important

Also it would be good if you implement some archiving solution in place for emails older than 10 months so that your primary mailbox will not be loaded always and less load on the exchange server.
When you bring up archive make sure that the retention for the archived items is planned properly (depending upon your client suggestion) as few orgs would like to have emails of more than 5 years since the retention of the archive cannot be restored at any cost unless we have alternative solution in that place.

Planning this part and convincing the client is little bit tough and difficult since the cost is involved more in this factor when they say they want to keep the retention items in archive for ever.

Hope this reading is helpful

Thanks

Sathish Veerapandian

MVP- Exchange Server