Users of Microsoft Outlook receive 15 GB of free email storage per account. However, loads of emails, especially with attachments, often cause the storage to reach its limit. If your mailbox is full, you can no longer send or receive any new message.
When this scenario happens, the best thing you can do is to clean up your mailbox. Freeing up your storage space from unwanted messages and files will give room to incoming ones and enable you to send out emails. At the same time, it is also ideal to identify what may be causing your mailbox to maximize its storage for you to avoid the same problem in the future.
Effective Ways to Cleanup Storage and Reclaim Precious Space for your Outlook Mailbox

Several methods can help you delete unnecessary files and messages in your mailbox and free up storage space. Let’s check them out below.
Empty your Deleted Items Folder
First, empty your Deleted Items folder. These files are in the Deleted Items folder for a reason – you want to delete them. However, having these messages remained in the folder still counts with the storage limit. You can do this manually by right-clicking the Deleted Items folder. Click Empty Folder.
If you want Outlook to automatically delete all items from the folder, you can set it via Properties. Right-click the Deleted Items folder and select Properties. Go to AutoArchive and click Archive This Folder Using These Settings. Now, set how old the items should be and select Permanently Delete Old Items. Click Apply and click OK.
Save All your Attachments to your OneDrive Account

Next method is to save all attachments to your OneDrive account. Not only will it free up storage space that is taken up by big attachment documents, images, or videos, you can also store and manage these files in one place. First, filter your messages by sizes so that you can select those with large attachments.
Go to Filter above your message list, hit Sort by and select Size. Next, choose the messages you want to save to OneDrive. Click Save to OneDrive.
If you choose to save the attachments to your computer, open the email message and go to the attachment. Click the down arrow next to it and select Save As or Save All Attachments. Choose the folder where you want to save the file and click Save.
Empty your Spam or Junk Mail Folder
Let’s not forget your junk mail. These spam emails are not needed in your mailbox but can be the reason why you no longer have storage space. Try to review your Junk emails and empty the folder.
Clean up your Old Emails
Another way is to archive old emails. Go to File and then select Options. Click Advanced and head over to AutoArchive. Select AutoArchive Settings. Click Run AutoArchive Every (blank) Days and set the number of days. Next, edit the During AutoArchive section. Once done, click OK.
Lastly, you can use the Mailbox Cleanup. Hit the File button. Go to Info and select Cleanup Tools. Now click Mailbox Cleanup. Now, set the options according to your preference.
Wrapping It Up
It may be challenging to manage all your emails if you have been receiving tons every week. However, you can keep it clean once in a while by following the tips above and make sure you only have what you need in your Outlook mailbox.