Question for Techies

i use Yahoo as my main email address. i just started using Outlook with it. i check my email on my computer and my phone. My problem is that when messages go on outlook, they are erased off Yahoo. So for instance, a few weeks ago i needed some an address someone had emailed me for my gps. No problem. Jumped on my phone, got it, and was there in 20 min. But now i no longer have access to it on my phone or other computers. What do i do?