Way back in the dark ages, when Exchange Server was originally released, every Exchange database had a limit of 16 GB. That didn’t last long. By Exchange 5.5, Microsoft had split the Exchange product into a Standard Edition and an Enterprise Edition. The Standard Edition retained the 16 GB limit, but the Enterprise Edition could have databases of any size. Of course, the practicality of this was limited by the speed of backup and recovery available and the SLAs (Service Level Agreements) that an IT organization had with its customers.
Starting with Exchange 2000 Server, Microsoft allowed the Enterprise Edition of Exchange Server to have up to twenty mounted mailbox databases per server (with the Standard Edition still limited to a single mailbox database per server). This particular limit stayed the same in Exchange Server 2003. In Exchange Server 2007, the Standard Edition can now have five mounted mailbox databases and the Enterprise Edition may have up to fifty mounted mailbox databases.
Another important change happened for Standard Edition beginning with Exchange Server 2003 service pack 2. As of that service pack, Microsoft removed the 16 GB limit from the mailbox store and allowed it to grow as large as 75 GB (with a registry change – see KB 912375 - How to increase the Exchange Server 2003 Service Pack 2 18-gigabyte database size limit). In Exchange Server 2007, the database size limit was removed from Standard Edition completely.
Now, with Exchange Server either edition, the size of your mailbox databases is limited by your backup and restore timeframe. Remember, if you have to do a mailbox database restore to a live database – your users are down while the restore is happening and while the log files are replaying. Downtime should be minimized as much as possible.
So, while you should always protect yourself by taking database backups, and you should always verify that you have good backups, the real goal is to only have to use those backups in a “disaster recovery” situation. That is, heaven forbid, your data center is flooded, or something physically destroys your Exchange Server.
Exchange and Outlook, working together, provide mechanisms for assisting in reaching this goal. They are called Deleted Item Retention and Deleted Mailbox Retention. These values are configured on each mailbox database. With these features and a little user and administrator training, you will probably never have to execute that database restore!
Whether using ESM (for Exchange Server 2003) or EMC (for Exchange Server 2007), you set these attributes on the property sheet for a specific mailbox database. I recommend that you set both values to 30 days, if possible, but for a minimum of 7 days.
Note that once you set these attributes, that items are not physically deleted from your mailbox database until after XX days have passed, where XX is the number of days for which you configure the attributes. This can cause the disk space requirements for your mailbox database to significantly expand, as well as increase the size of your backup media requirements.
Finally, if your users are using Outlook 2003 with RPC/HTTP, you should refer to KB 886205 - Deleted items are not available after you use "Recover Deleted Items" in Outlook 2003, and make the change recommended there.
Using the Recover Deleted Items functionality is easy. An overview of it (which applies to Outlook 2003 and Outlook 2007) is available at https://technet.microsoft.com/en-us/library/aa997155.aspx.
While there are no guarantees that this will always work, using this technique should minimize the necessity for ever restoring that mailbox database from backup.
Overview of Recover Deleted Items functionality (applies to Outlook 2003 and Outlook 2007)
How to increase the Exchange Server 2003 Service Pack 2 18-gigabyte database size limit
Deleted items are not available after you use "Recover Deleted Items" in Outlook 2003