E-mail Disappearing from iPhone 6 iOS 9

Ever since I upgraded my iOS to 9.0, my iPhone 6 has been removing the content of my emails once I download from off the Comcast server onto my home computer. They used to stay there (on the phone) until I manually deleted them. Now, although they stay in the list of items in my Inbox, when I tap one to read it, I get the message "This message has not been downloaded from the server." Well, it has been downloaded from the server, it's just not there on my phone anymore. Anyone know how to make my phone stop doing this? I am using a POP account and do NOT want to use IMAP. IMAP would delete items off my home computer if I deleted them from my phone, and vice versa. Great if you want everything synchronized, but that is not my goal. I want to keep everything on my home computer an only certain, fairly recent items (like receipts for airline tickets) on my iPhone. Thanks for any help.