Email is still one of the main tools most businesses rely on, and Microsoft Outlook is where many people manage it day to day.
But as messages, attachments and folders build up, Outlook mailboxes can become slow, hard to search and more likely to run into sync problems. Managing mailbox size properly helps keep Outlook usable.
This article covers practical ways to keep Outlook mailboxes in your business under control. We’ll look at reducing unnecessary storage, organising messages sensibly, and using Outlook’s built-in tools to keep mailboxes manageable.
By following these practices, you can make Outlook easier to use and reduce the problems caused by an overloaded mailbox.
The problem
For many people, Outlook isn’t just used to send and receive email. It also becomes a long-term store for email conversations.
As your mailbox grows, searching through it can become increasingly difficult and time-consuming. It can also put more pressure on the computer, storage and network connection.
You may not realise that desktop Outlook synchronises a local copy of the mailbox with the main version stored in Microsoft 365 or on your mail server. The more emails and folders to synchronise, the more chance for things to go wrong with the sync process too.
These problems can become worse if you also use shared mailboxes, open colleagues’ mailboxes, or regularly send and receive large attachments.
Best practices for Outlook mailbox size
Below are some of the techniques we use for clients when Outlook mailbox size starts causing problems.
Mailbox Cleanup
This is a good starting point for when you first start to experience problems.
In Outlook, choose File > Tools > Mailbox Cleanup
From here you can view the size of your mailbox and its folders, find items older than a certain date or larger than a certain size and archive items by using AutoArchive.
Improving performance when you are connected to many shared mailboxes
If Outlook is slow or starts throwing errors when you have a lot of connected mailboxes, you may be hitting a limit on what is possible with your current setup. This can be caused by older or slower PCs and laptops, too many large mailboxes, or a slow network or internet connection.
As a general rule, if you have more than 5 mailboxes attached to Outlook, you may start to see sync and speed issues more frequently. More than 10 and you may find Outlook unusable.
If you do need to access all these mailboxes, consider other ways of doing it – for example, using online Outlook in Microsoft 365.
Microsoft has a detailed guide for this.
Handling Quota Exceeded messages
At time of writing, standard Outlook accounts in Microsoft 365 can have up to 100GB of storage per mailbox. This is a large mailbox, but it’s still possible to fill it, particularly if you send or receive large files or have accumulated many years of email.
If your mailbox is nearing capacity, you may start to receive “Quota Exceeded, your account is above quota limits” messages, either in the app as pop-up messages or as emails.
If this happens, you can see how much storage you’ve used and quickly clear up space by going here for all Outlook versions prior to 2024:
Settings > General > Storage
Or here for New Outlook for Windows:
Settings > Accounts > Storage
From the Storage page, you should see a breakdown of how much space each folder in your mailbox is taking, along with various options to permanently delete items to free up space.
As an IT company, there may be things we can do to increase quota. But that only buys time before the extra space fills up too.
Archive older items automatically
If you must keep every email, archiving older items is the sensible long-term answer. By automatically removing emails from your mailboxes older than a certain age, and storing them somewhere else, you can keep your primary mailboxes operating efficiently.
Microsoft explains how to do this for individual mailboxes here.
Better still is to have a company-wide archiving approach in place. There are several ways to do this, including higher Microsoft 365 licence tiers with more archiving options than basic auto-archive. There are also third-party tools that can manage archiving for you. Please contact us if you need help deciding what is suitable.
Conclusion
Managing Outlook mailbox size is not just about tidiness. It can help Outlook run better, make emails easier to find and reduce avoidable problems for your team.