Email encryption options on an iPhone
Try iPGMail.

More here.

Features (copied from iTunes app store):


  PGMail is an app that implements the OpenPGP standard (RFC 4880) and
  allows the user to create and manage both public and private (RSA and
  DSA) PGP keys and send and receive PGP encrypted messages.
  
  PGP Keys and Messages can be passed to iPGMail several ways:
  
  
  From the iOS Mail.app - The iOS mail application will pass PGP attachments to iPGMail directly, eliminating the need to do an awkward
  copy-and-paste. NOTE: PGP messages that are part of the main body of
  an email will still have to be copy-and-pasted, only properly tagged
  attachments can be passed automatically.
  Directly from the system clipboard. Copy-and-paste the PGP message text from any file, webpage, or message, and it can be imported and
  decrypted by the app.
  iTunes File Sharing - Connect your device to a computer and open up iTunes, in the "Apps" section, you can scroll down and select iPGMail
  under the File Sharing section and transfer files to and from the app
  to your computer.
  DropBox - Starting with iPGMail v 1.14, you can link iPGMail to a dropbox account and transfer files to and from your dropbox file
  space. This means you can encrypt and protect your dropbox files or
  share files with others through your existing dropbox Public
  interface.
  
  
  iPGMail supports PGP key generation (RSA) directly in the app and
  stores them securely using the iOS keychain for the application. Using
  your own private PGP keys, you can encrypt and/or digitally sign any
  messages. Optionally, your public key can be attached to any message
  you send from the app so that the recipient can then import it into
  their own keychain, either on the phone or on any computer with PGP
  support.
  
  iPGMail will import your private keys so you can reuse your existing
  PGP identity and keys on your iPhone or iPad. Both public and private
  keys can be imported through the interfaces listed above.
  
  iPGMail allows the user to search public SKS PGP keyservers to find
  registered public keys for other people with whom the user can then
  send email that is digitally signed and/or encrypted encoded in
  OpenPGP ASCII Armor.
  
  This app is ideal for securing your files or for sending secure email
  messages to specific parties without worrying about it being viewed by
  anyone other than the designated recipient.