Managing mailbox storage use in Exchange 2010, Part 1
It's something most organizations encounter at some point: A mail server with no more space left or user's can't get a higher quota due to limited storage resources. Even with a basic policy regarding mailbox quota's, running out of resources is still something that can happen if the server was not sized properly or upgraded to handle changed conditions (larger quota's, more mailboxes etc.).
Most straightforward would be upgrading the storage solution, but there are other options that could be a better fit for your organization. In this series of posts I will summarize the most common solutions I have encountered, implemented or even advised against.
PST
The Outlook Personal Folders file (or Personal Store with extension PST) are a very commonly used under these circumstances. Users often can make them without restrictions. An admin has to make effort to prohibit usage and there are also no extra licenses necessary, which means that there is no real incentive to restrict it at first. However, as time has probably taught most (Exchange) admins by now, there are a lot of reasons NOT to use PST files:
- Only available via an Outlook client (and only one at a time), not via ActiveSync devices and/or Outlook Web App.
- There is no oversight on storage usage, although when placed on an fileserver a quota could be implemented but:
- Stored on a network share is not a supported situation by Microsoft (See KB297019; problems with shares and slower performance are important reasons)
- Stored on local machines limits availability to exclusively that machine.
- Backup could be problematic while in use and/or when on local machine (most synchronization tools ignore PST's, including Offline Folders).
- Chance of theft (when on laptop) and corruption and thus loss of complete content of PST file.
- There is no uniform search function, searching for mail has to be performed explicitly within PST.
Just don't use PSTs. In my experience a lot of Service desk time will be spent resolving some of the issues resulting from their use. It’s probably the IT department were the term ‘PST Hell’ was coined first. 
You can use them for permanent export of mailboxes or as an migration intermediate. But not as a day-to-data mailbox extension. It is possible to restrict the use of PST's via registry settings or Group Policies.
On-premises Personal Archive
A new feature of Exchange 2010 is the Personal Archive function, not to be confused with Auto-archiving within Outlook. Basically, users get a second mailbox on Exchange with a separate quota. That Archive mailbox is available while connected to Exchange via Outlook (2007 & 2010) and Outlook Web App. It was actually devised as a possible solution to the aforementioned ‘PST hell’. More general info here.
Benefits are:
- Look and feel of PST, so if PST were used users are used to it
- Available via Outlook Web App.
- Uniform search (one search query for main and archive mailbox).
- Management of Archive mailboxes basically the same, so relatively low impact.
- With Exchange 2010 Service Pack 1 Archive Mailbox can be placed in other database and thus other (cheaper) storage solution than main mailbox (i.e. storage tiering). This also make a different RTO/RPO possible (lower backup frequency, no redundancy via DAG etc. etc.)
- Admins can control the retention and archiving of items via Retention Policies and give some control to users via Personal Tags.
But there are some drawbacks as well:
- Two locations within Outlook (Web App) which contain mail instead of one central mailbox.
- Although users can drag-drop items, automation policies are retention (time) based. User could still be confronted by his/her quota and will have to move items manually.
- Only available when online (i.e. there is no caching in Outlook, this could also seen as a benefit BTW).
- No compliancy enforcement, user are still able to delete/edit mail items.
- No access via ActiveSync devices.
- Optimal use only with Outlook 2010, Outlook 2007 has access with correct patches.
- Office Professional Plus is needed.
- Exchange Enterprise CAL needed per user with Archive Mailbox.
Only since Exchange 2010 Service Pack 1 the Archive Mailbox became IMHO a viable tool for Exchange storage management, due of the option of placing it in another database. It adds flexibility in a lot of scenarios. Possibly preventing the need for a storage upgrade, for instance when the Archive Mailboxes are moved to a Database not made redundant via DAG.
There are possible costs involved depending on you situation, you still need storage (but possible less) and Enterprise CALs (which could already be in possession). Each organization has to determine whether this method is one they would appreciate.
It's easy to implement, just click Enable Archive on the user object within the Exchange Management Console (EMC). Other management of those mailboxes is basically the same as normal mailboxes. The use of Retention Policies and Personal Tags is somewhat more complex however, but implementation other than the default policy settings is probably only a one time thing.
Concluding
We've discussed issues with storage use within Exchange and discussed the pro/cons of PST files and Online Archive. This concludes part 1 of this 3 part series of posts. The next post will discuss third party archiving solutions and Exchange Online Archiving.
Managing mailbox storage use in Exchange 2010, Part 2
Managing mailbox storage use in Exchange 2010, Part 3