Android phones like the Samsung Galaxy Y S5360 have just 168 mb of RAM available for use. If you have multiple email accounts and want to read email on the go using the phone, then you have no choice but to download the GMail client (provided by default) and then to read Yahoo mail, download and install the Yahoo Mail client from the Google Play Store. If you also have Hotmail or another email account with divergent SMTP/POP settings, then you need to find an email client to enable you to read those emails too on the go.
Luckily for android users, there is a solution. You can install K-9 email client from the Google Play Store and have all emails in one email client from different email service providers or servers. This saves you the installation space that would have been taken up by multiple email clients for each service provider that you use. By the way, Yahoo Mail client from the play store installs into the phone memory rather than the the external storage memory.
You can free up memory on the phone by rooting it, but that is another topic altogether.