Depends on phone

You can usually kill all the apps and that should log you out. If not go all the way to the top right hand corner there should be an option under your username click on it and click log out. If not go to phone settings under mail or internet connection and delete all history that too should log you out, make sure to unclick save password that way no one can log in if your phone is unattended.  

Hopes that helps
xoxo

uust click on your name and it should drop down a menu and you will see where you can log out there. I also use yahok but i switch back to the classic mail which can be found in the options menu.

If so, do what I did:

Have a provider set it up for you.

I have Yahoo email also, but could never figure out how to get on it with the iPhone.  I had a gal over one night and told her my problem.  She took my phone and in about 2 minutes set it up so the Yahoo Mail is always there when I want it.  It uses something called an exchange, as I recall.

Providers know everything there is to know about computers.  I'd be lost without them.  And they look a lot better than most techs too

All those "free" email services, yahoo, gmail, hotmail, etc. will gladly give up ALL of your email records to LE when asked. The smart thing is to use a foreign email service that can't be pressured by American LE. Just my 2 cents.