If your mailbox is getting near capacity you may receive a message that looks like this
You will receive that message daily until some email items have been deleted from outlook. The number of items deleted doesn't matter so much as the size of each item. 1,000 emails could be about equivalent to just 2 or 3 of the largest emails.
This limit includes everything in your mailbox including
One of the fastest ways to free up space is by using the Search Folders feature to show the largest items.
Search Folders can be found in the folder list near the bottom. The sub Folder you're looking will be labelled as either Large Mail, or Larger than ###
If you see a Search Folders, but don't have a sub-folder labelled 'Large Mail' or 'Larger than ###' then right click on the Search Folders and select 'New Search Folder...' Then scroll down and select Large Mail from the list and click OK.
Once viewing the Large Mail folder, click on the SIZE header as shown below to sort items from largest to smallest. 1 MB = 1024KB, so if you see a 950KB item, it's still smaller than a 1MB item.
Then start from the top and work your way down. Determine which items can be deleted, which need to be saved first before being deleted, and delete them.
Don't forget to also delete them from your Deleted Items, or empty your Deleted Items folder as well.
The example shown above is a mild example. The 8 items listed add up to only 28MB, or roughly 1% of the overall quota. On some mailboxes the top 8 items can easily add up to 10-20% of the entire quota 200-400MB. This is due to large attachments, typically video files, pictures, or some powerpoint presentations.