Large mailbox sizes and no mailbox limit

Large mailbox sizes and no mailbox limit bring three main concerns:

1) Time to recovery and maintenance – which was improved in Exchange 2007, where you can have up to 50 storage groups with 50 databases, and of course, the asynchronous log shipping technology (LCR, CCR, SCR) where you can dramatically improve the recovery time objective (RTO) for the provided messaging services and the recovery point objective (RPO) for the provided data.

2) Performance – In fact, it is not the size of the mailbox that impacts performance, but the number of items per critical folders: Calendar, Contacts, Inbox and Sent Item folder. We recommend to keep the maximum of 5,00 items per critical folders (Outlook 2003). With Exchange 2007 and Outlook 2007 you can have up to 20,00o items per critical folder. I have seen customers with 70,000 items, and of course, slowness issues.

3) Security – It is really hard to minimize the possibility and prevent denial-of-service attacks with no mailbox size limits (the sky is the limit). An attack of this type can be originated from sending a large number of e-mail messages to a particular server until it runs out of disk space or cause a database corruption.

Implications in having 20GB mailboxes (or very large mailboxes)

Performance issues at client side – Outlook clients with very large .OST files (2GB is already large) may experience slowness issues on desktop, but more on portable computers. The reason for that is the hard disks speed limitation on the portable computers, generally 5,400 RPM. And since the performance problems will occur directly on the hard disk, there is no way to mitigate that adding more resource, as memory RAM, for instance. Just a remind: Outlook PST files are not supported on network file shares.

Performance issues at server side – disable the cache mode and run the Outlook client in online mode will moves the IO load from the client to the server, and now you have two problems: there is no “offline mode” and you have to plan to support the total IO throughput caused by the heavy users.

But even these performance issues can be addressed. The best solution to support very large mailboxes is to limit the size of the OST files. On Exchange 2007 you can use the Messaging Records Management (MRM) policies to “archive” old items and then configure the Outlook 2007 to not sync the “archive” content. This way the “critical” or “most used” content will be always synchronized and the archived items will be available online. See reference link “Plan a Cached Exchange Mode deployment in Outlook 2007”.

Why eliminate PST files?

Security and reliability risks

There are many costs and risks associated with PST file management. The end-user ability to have local large PST files with unlimited and informal recovery guarantee is something the any IT department will be unable to accomplish. And this is not due to  limited IT resources or the ability to guarantee the availability of end-user offline data, but mainly because of the impossibility to manage the “unknown” offline content.

Exchange mail databases commonly contains a wide range of vital corporate data such as personal data, customer data, product plans, marketing plans, corporate financial data, intellectual property, etc. In addition to security risk to lose all these data (CD, DVD, stolen laptops, etc.), there is no archiving strategy and benefit in maintain the critical data in PST files as the company will not be able to find and recovery any information when needed.

But, if the customer wants to keep PST files, my recommendation is to have in place an established SLA (Service Level Agreement) between IT departments and the business units, describing which kind of recovery scenarios they should reasonably expect for offline data, taking into account their business needs and their IT infra-structure.


Mailbox Server Storage Design (there is a specific section for mailbox sizing guidance – Actual Mailbox Size)

High Availability Strategies

Plan a Cached Exchange Mode deployment in Outlook 2007

Recommended Mailbox Size Limits

Optimizing Outlook 2007 Cache Mode Performance for a Very Large Mailbox
This is a very good article by Matt Gossage – SENIOR PROGRAM MANAGER – showing how to optimize performance at client level for large mailboxes (his mailbox size is 7.4GB)

You may experience performance problems when you are working with items in a large .pst file or in a large .ost file in Outlook 2007
Anderson Gobbi