Microsoft Outlook places limits on the size of your mailbox. The appointments and notes in your calendar are included with your email messages in calculating the mailbox size. Removing some of the items you no longer need will reduce the size of your calendar and give you more room for email messages.
Sorting Your Calendar Items
Sort your items by category to make it easier to search through your calendar. Select "View" from the top menu bar, click "Arrange by" and choose "Categories." Check the ending date before deleting any recurring appointments. Deleting one instance of a recurring event will also remove all future appointments related to that event.
Deciding Which Items to Remove
You can also sort your calendar items by size and simply start removing the largest ones until you have freed up enough space. To add a column showing the size of each calendar item, click the "Fields" button, select "All Mail" and choose "Size" from the list of available fields.
Other Ways to Save Space
Once you have removed the unnecessary appointments and invitations, look through your mailbox for messages related to those events. Delete the messages that you will not need to reference again in the future. If you are saving a message solely for that person's contact information, take the time to set up a new contact instead and delete the message from your mailbox.