I am looking for an email client, that when opened, asks for a password before showing email messages. Is there such an app available for iOS?
What kind of server are you accessing? If you use Exchange, check out Mail+ for Outlook. They have an easy to set passcode option which prompts you for a pin each time the app is launched. If you do not successfully provide the passcode within 5 attempts, your credentials and all emails are erased.
If you are using ActiveSync (or have ActiveSync for Gmail), check out their other version. It uses similar technology.
Boxer does this. There is a free and a paid version.
Unfortunatley it is, in my experience, buggy, and it doesn't support text expander touch. I'm still on the lookout for the perfect mail app.