If you're running low on disk space on your Mac, especially SSD space, every extra bit can count. One of the places you can often scrape out a few extra megabytes or even gigabytes is Mail Downloads. Any attachments you open in Mail or Quick Look gets saved right to that folder. Chances are you don't need them, especially the old ones, and trashing them will free up some valuable space.
If you use Apple's Mail app for OS X and have multiple accounts set up, you may have noticed some lagging issues, particularly if you're running Lion or Mountain Lion. I have been experiencing severe issues where Mail.app takes several seconds to catch up with what I am typing or performs poorly when loading messages which results in excruciatingly long wait times. One of my colleagues shared an interesting piece with me by Brett Terpstra that fixed it for me in just a few steps:
If you use the default Mail app on your Mac and you're running OS X Mountain Lion, you can add a sender to your VIP list just as easily as you can on your iPhone or iPad. VIP list basically will filter out emails you want based on who is sending them, therefore making finding messages a lot easier.
Don't know how to use VIP Inbox on your Mac? Here's how: