The iPhone lets you open and listen to MP3s sent as email attachments, but not to save songs directly to the music app on your iPhone. To move songs that were sent as email attachments onto your iPhone, you must first download them on your computer and then sync via iTunes.
Items you will need
USB dock connector
Computer access to iPhone email account
Windows or Mac computer
Open your email account using the Windows or Mac computer that houses your iTunes library.
Open the email to which the MP3 file is attached.
Download the MP3 file to your computer hard disk. Take note of the file name and folder location on the hard disk.
Click “Add File to Library” if you are on a Windows machine, or “Add to Library” if you are on a Mac.
Search for and select the MP3 file you downloaded in Step 3, then click “Open.”
Connect your iPhone to your computer via a Universal Serial Bus (USB) dock connector and synchronize.
Apple: iPhone User Guide