Applying storage quotas to Exchange Server mailboxes is a best practice.
The trend for Exchange Server in recent years is towards larger mailboxes. Email is a heavily used application, users are demanding more storage for their email, and current versions of Exchange Server have been engineered to provide large mailboxes without requiring expensive, high performance storage subsystems to store it all.
All of that does not mean that you should remove storage quotas for your mailboxes. Unrestricted growth for mailboxes is a risk to your Exchange environment, especially when capacity monitoring and planning is not occurring (which, let's face it, is the case for most organizations). Mailbox storage quotas prevent mailboxes from growing beyond your ability to provide capacity, and perform management tasks such as backup/restore, or mailbox migrations.
Consider a scenario in which a user departs the organization. It is common for departed user mailboxes to be left active, continuing to receive new email even when they have been removed from internal distribution lists. Eventually, a large number of departed user mailboxes can accumulate, consuming enough disk space to cause a problem.
In extreme cases, a single mailbox could consume all available space on a database volume. Many Exchange admins will be familiar with at least one case in their own experience in which a gigantic mailbox was discovered. Common causes of this are:
- Shared mailboxes for teams that insist on keeping everything in the shared mailbox forever.
- “Monitoring” mailboxes that someone has used for collecting email alerts from an application, and then forgotten to turn them off when they no longer needed them.
- Journal mailboxes.
Exchange Online has storage quotas for mailboxes. The quota size depends on the plan you're subscribed to. Some plans also include “unlimited” archive mailboxes. The unlimited archive quota is achieved by chaining together 50GB archive mailboxes, referred to as auto-expanding archives, which allows for “unlimited” size without the risk of a single, large archive mailbox causing a problem within the service.
So if mailbox quotas are a best practice, what should the quotas be set to? That question does not have one single answer that would fit all organizations. Storage quotas will be influenced by a variety of factors, such as:
- Number of servers, databases, users…
- Available disk storage (and budget to buy more)
- Use of archiving solutions
- Demand for exemptions (for special mailboxes or VIPs)
You can read more about managing Exchange Server mailbox quotas here.