I'll try to help you out some here. I'm assuming you're using hotmail to send the attachment. Unless you are paying for extra space you have only 2 MB of space on your account which will limit the size of attachments you can send and how many messages you can keep in there.
What kind of attachment (text, photos, graphics) are you trying to send? When sending photos and graphics always be sure to resize and convert them to .jpg format first. That will cut down on the size of the file you want to send. Sending them in .bmp format can eat up your space in a hurry. (All photos posted to groups must be in .jpg format since each group member is allowed only 3 MB per passport. Text does not count toward that allocation - only photos and attachments count.)
If it's a very large text file, you may have to send it in segments in separate e-mails.
Keep your hotmail account cleaned up. Delete any e-mails you don't need. If you really don't want to toss some of those messages you can copy and paste them to a WordPad or Notepad document then delete them from your mailbox. You can also set up a second hotmail account and forward them - use this account for this purpose only (do not give out the address). Make folders to keep them organized. Keep in mind that if you do this you must check that account regularly or MSN will delete it. The spam will fill it up quickly so it's a good idea to set your filter to Exclusive and access it every few days and clean it out.
Now for the cache cleanup. This is not for your e-mail but for the benefit of your computer. This should be done after finishing each web-surfing session. If I'm online for very long I clear my cache several times during the session. I can only give you instructions for Internet Explorer (IE) though. Before closing IE, go to Tools at the top of the browser. Click on Internet Options. Under Temporary Internet Files click on Settings then on View Files. On your toolbar at the top click on Edit then on Select All. All the files on that screen will now be highlighted. On the same toolbar click on File then on Delete. You will get a warning asking if you want to delete the selected cookies. Tell it Yes and all those temporary internet files will disappear leaving your cache cleared. Click on Ok then OK again to close the Internet Options dialog box. You can now close your browser.
I hope this helps. If you need more assistance you can e-mail me, or better yet, catch me on Instant Messenger and I can walk you through the process.