Yahoo email question

I have yahoo email that I check from my phone and computer.  When I wake up in the morning I will have all my new messages as well as all my read messages on my phone.  When I do anything with my email on a computer, it wipes out all my old messages and usually keeps the new messages on my phone.  Is there any way to keep all my emails on my phone at all times?  I went through the settings and check everything and there doesn't seem to be any settings to change.  Is this a yahoo issue or phone issue?  Thanks for the help.

I think I figured it out.  If I open an email with my phone, they stay on the phone.  If I delete any on my comp they all disappear off my phone except for the ones I previously opened on my phone.  This sucks!

This really makes me mad that my emails get deleted from my phone with no good reason, especially when I open them from my phone!?!!  The only other solution that I have I thought of is to open all my mail on my phone.  This is my only major (other than the lack of apps) that I have with this phone.