How to increase mailbox size in Office 365: A step-by-step guide

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Managing your Office 365 mailbox size effectively is crucial to ensuring smooth email operations and avoiding any interruptions in communication.  

This guide will walk you through the steps to increase your mailbox size in Office 365 and offer some best practices for maintaining a healthy mailbox. 

Understanding the Office 365 mailbox storage limit 

Office 365 mailboxes come with predefined storage limits, which vary based on your subscription plan. Typically, standard Office 365 mailboxes have a limit of 50GB for Business Basic and Standard plans, while Premium and Enterprise plans offer 100GB.  

Exceeding your mailbox quota can result in bounced emails, reduced performance, and other operational issues. Therefore, it's essential to monitor and manage your mailbox size proactively. 

Common issues with Office 365 mailbox storage 

Businesses often hit their Office 365 mailbox storage limit swiftly due to the high volume of daily email exchanges.

Key challenges related to Office 365 mailbox storage include capacity restrictions, management hurdles, and performance concerns.

Here are the main issues that users and administrators encounter:

1. Mailbox quota limits and overages 

Office 365 plans come with specific storage limits for mailboxes. Users often encounter issues when they exceed these limits. When a mailbox is nearing its storage limit, warning notifications are sent, but they’re sometimes missed, leading to blocked sending/receiving of emails. 

Shared mailboxes and archive mailboxes also have their own storage limits, and exceeding these limits can cause service interruptions. 

2. Retention policies and storage management 

Misconfigured or overly aggressive retention policies can cause older emails or items to be deleted automatically, leading to data loss. Archive mailboxes are meant to store older emails, but some users face issues with automatic archiving not working correctly, causing primary mailboxes to fill up. 

Specific folders like the "Sent Items" and "Deleted Items" can grow large over time if not properly managed, taking up significant storage space. 

3. Performance degradation 

As mailboxes grow larger, users may experience delays or sluggish performance when accessing email, especially with older emails. When mailboxes are near capacity or poorly indexed, search performance can degrade, leading to slow or incomplete search results. 

4. Synchronization issues with local storage (OST files) 

Outlook uses an Offline Storage Table (OST) file to cache mailbox data locally. If the mailbox size exceeds the OST’s practical capacity, synchronization issues can occur, affecting email delivery and access. 

Large or corrupt OST files can lead to synchronization problems, where email items fail to appear in Outlook, even though they exist in the web version of Office 365. 

5. Shared and delegate mailbox storage management 

Shared mailboxes often have multiple users, making it challenging to manage storage efficiently. If a shared mailbox exceeds its limit, it can prevent all users from sending/receiving emails. 

Issues can arise when a user with delegate access adds multiple mailboxes in their Outlook profile, leading to increased OST file size and performance issues. 

6. Compliance and legal hold storage 

Mailboxes under litigation hold or eDiscovery can accumulate large amounts of data since nothing is deleted, quickly consuming storage space. 

If the in-place archive is not properly used, it can lead to primary mailbox filling up, causing quota issues. 

7. Auto-expanding archives not functioning 

Office 365 offers auto-expanding archives for users who exceed 100 GB, but sometimes this feature doesn’t kick in as expected, leading to storage issues. 

8. Misconfigured or overlooked mail flow rules 

Sometimes, mail flow rules are configured incorrectly, causing an unexpected accumulation of emails in certain mailboxes, leading to storage overages. 

9. End-user mismanagement 

Users often store large files in their mailbox instead of using OneDrive or SharePoint, leading to unnecessary mailbox bloat. 

Some users forget to clear their "Deleted Items" folder, which continues to use mailbox storage. 

Ways to increase mailbox size in Office 365 

There are several methods to manage and increase your Office 365 mailbox size. Here are some effective strategies: 

1. Archiving mailbox content 

Archiving old emails is a great way to free up space in your mailbox. Here are the steps to enable auto-archive: 

1. Open Outlook and go to File > Options. 

   
   

2. Select Advanced and click on AutoArchive Settings. 

   
   

3. Choose how often you want to run AutoArchive. 

   
   

4. Select the items you want to archive and specify their age. 

   
   

5. Choose the location where you want to store the archived items. 

   
   

6. Click OK to save your settings. 

Here’s a full guide on how to automatically remove older emails from Exchange and Outlook. 

2. Requesting a mailbox size limit increase 

Administrators can request an increase in the Office 365 mailbox size limit in specific scenarios, such as for senior executives or heavy email users. Here are the steps to request a size increase: 

1. Log in to the Microsoft 365 Admin Center. 

   
   

2. Go to Admin Centers > Exchange. 

   
   

3. Select the user whose mailbox size you want to increase. 

   
   

4. Click on Mailbox Features. 

   
   

5. Under Mailbox Storage, click Enable for the Archive option. 

   
   

6. Save the settings, and the mailbox size will increase automatically. 

3. Using shared mailboxes 

Shared mailboxes are an efficient way to expand your Office 365 mailbox capabilities without additional licensing costs. Here are the steps to set up a shared mailbox: 

1. Log in to the Microsoft 365 Admin Center. 

   
   

2. Navigate to Groups > Shared Mailboxes. 

   
   

3. Click Add a Mailbox and enter the required details. 

   
   

4. Assign members who will have access to this shared mailbox. 

   
   

5. Click Save and configure the mailbox as needed. 

Best practices for mailbox management 

As they say, prevention is better than cure. So, to proactively maintain a healthy mailbox size, consider these best practices: 

1. Regularly review sent and deleted items: Clean out unnecessary emails periodically. 

   
   

2. Use folders and labels: Organize your emails into folders to make management easier. 

   
   

3. Optimize attachments: Use cloud storage for large files instead of attaching them to emails. 

   
   

4. Set up rules for automatic sorting and deletion: Create rules to automatically sort and delete emails based on your preferences. 

End your Office 365 mailbox woes 

Managing your Office 365 mailbox size is critical to ensuring seamless email communication within your organization.

By following the steps outlined above, you can effectively increase your mailbox size and keep your operations running smoothly.