In this post, we're going to walk you through four ways to reduce the Outlook mailbox size to resolve Outlook mailbox full error issue. To fix the error, you need to minimize the mailbox size in your Outlook. Well, this error message occurs when the Outlook PST file reached its maximum storage limit. View complete answer on Why is Hotmail saying my mailbox is full? Either remove the attachments, move attachments to separate storage or delete messages with large attachments. The fastest way to reduce your mailbox size is to focus on mail attachments in the Inbox and Sent Items. What to do if my Hotmail account is full? How do I reduce the size of my Outlook mailbox without deleting it?.Why does my Outlook keep saying my mailbox is full?.What is the easiest way to clean up Outlook Inbox?.Why is it saying my mailbox is full email?.How do I know if my Hotmail mailbox is full?.What to do when Hotmail mailbox is full?.How do I clean up thousands of emails in Hotmail?.What do I do when my Hotmail mailbox is full?.How do I free up storage space on Hotmail?.How do I clean up my Outlook mailbox full?.Why is Hotmail saying my mailbox is full?.What to do if my Hotmail account is full?.