Hello, @IniobongNkanga-8038,
Welcome to the Microsoft Q&A platform!
Increasing mailbox size usually depends on the type of Outlook account and subscription plan you use.
There are a couple of ways to increase your mailbox size in Outlook, depending on your current license:
1.Upgrade to Exchange Online Plan 2: This plan increases the mailbox storage limit from 50 GB to 100 GB. You can upgrade through the Microsoft 365 admin center.
2.Enable Exchange Online Archiving: This feature provides additional storage space. Depending on your license, it can give you an extra 50 GB or nearly unlimited space with auto-expanding archiving.
3.Clean Up Your Mailbox: Regularly deleting unnecessary emails and attachments can help manage your mailbox size. You can also use tools like the Sweep function in Outlook to quickly remove unwanted emails. For steps, please refer to https://support.microsoft.com/en-us/office/mailbox-storage-limits-in-outlook-f170fe90-b859-4034-bcda-e186fc6a26f5. Use the search feature in Outlook to find and delete older emails with large attachments. Here are the storage limits for your reference.
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Thank you for your support and understanding.
Best Wishes,
Alex Zhang