Wow! If your startup disk is full, then deleting email messages will not help you much unless you received scanned images of pages from the Gutenberg Bible. Your best best is to buy an external hard drive and to move your excess files to the new drive. You get the biggest bang for the buck by moving video files. Garageband support files also take up a lot of space.
The reason that this is important is that MacOS X's virtual memory system wants 10% of your startup drive's capacity available as free space. If your internal drive is small (<100 GB), then you need to replace it with a larger drive if you don't want this to be a recurring issue.