When an MP3 file is a bit of music, such as a single song, it's usually small enough to fit under the maximum file size allowed to send it over e-mail. But if you have an MP3 file that is something much longer, such as an audio book or a recorded conversation, the size may be too large and you'll need to explore other options to get your MP3 files to send as an e-mail.
The website Mediafire is one simple way to send MP3 over e-mail since you can simply upload the music to the site and then send the link to your friend through e-mail. Click on the option to upload files for free without being a member and then copy the link you see there and paste it into your e-mail. The website is simple to use, and you get a decent upload size for free.
Try out the site Dropbox for a different method for delivering your file over e-mail. This site allows you to create a share folder on your computer where you can drop MP3 files. Open the new Dropbox folder and then click and drag music into it from other folders on your computer. Once you send the link to your recipient over e-mail, they can then access the files themselves. The space available for free on Dropbox is usually generous.
Download and install the Megaupload program from the website in order to upload MP3 files your friends can download through links you send them in e-mail. This site requires software, but it gives you additional options over Mediafire.