Secure E-Mail App for Android?

My girlfriend is very security conscious. She just got an Android phone and would likes to check her e-mail using an app instead of browsing to her mail's website, except that all the e-mail apps we have looked at automatically store your password. Despite being able to password protect her entire phone, she would still feel more comfortable if the app itself required to you enter your password every time you wanted to check/read mail. (She doesn't care about instant notification of new mails.) Is there an Android app that gives you this option that works with Yahoo! mail?