If you receive a message saying your mailbox is full, here are some things you can do:

1. Utilize your personal folders to move sent items and inbox items off of your server space. This will move the items off of your mailbox space and onto your local harddrive. You will still be able to access them while on your local computer. Find detailed instructions here.
2. Remember that your email saves all of your sent items. Delete unneeded sent items or move them to your personal folders as mentioned above.
3. Often a mailbox can be full because of a few very large attachments. You can sort your email by size and eliminate or move emails with very large attachments. To do so, add the size field to your email columns by following the directions here. Next click on the word 'size' to sort the emails by size.
4. Finally, make sure to delete your deleted items. You can do this by right-clicking your 'Deleted Items' folder and selecting 'Empty Deleted Items Folder.'