There are plenty android applications in the market.
I am using K9 with APG (Android Privacy Guard) and I am very pleased.
K-9 Mail is a free and open source email client for Android devices, that integrates seamlessly with Android Privacy Guard.
Android Privacy Guard (APG) is a free and open source application that lets you encrypt, decrypt and sign files, messages or emails using Public Key Encryption (like OpenPGP) or encrypt/decrypt files or messages with symmetric encryption, securing them with a password.
The use of these two tools allows for easy encrypting and decrypting of email messages.