Message "has not been downloaded from server" and "cannot be downloaded from server". Comcast email

"Has not downloaded from server" ...appears daily when I try to re-read emails in inbox, or "sent" or "trash." I use a Comcast email on my iPad. After uploading IOS 9.1 and the previous version this message began appearing and now appears with increasing frequency. It happens on emails in IN BOX that I had previously read hours earlier...it can happen when I check my "sent" category just moments later and will say "cannot download from server." I went to the Apple Genius Bar, and they said to upload Xfinity app and get mail,from there. I did, but that made no difference.

 

I don't know if this is a similar problem, but here in the UK I have a POP3 mail account, and when I was checking my mail on my PC, it was immediately removing it from the server. Consequently, any mail I had already downloaded to my iPad would give me the same message you are receiving.
What I didn't realise was that there was an option on my POP3 account settings on the PC to leave the mail on the server for x number of days. So now the mail gets left on the server for 30 days before it is automatically deleted, and I get to keep the mail on my devices as well.

 

Get the same on my outlook account since the last update. I have two outlook accounts, one is an aliase. I can go from one to the other and it eventually downloads

 

Grrr. This "message can not be downloaded" is driving me crazy! I have airline tickets that I need to use for check in and they can not be downloaded! I've had to go to my computer, locate the email, and forward it to myself.... again! The apple tech had no idea what was happening. Really?
So, will the easy fix be to try my server settings? My server is Frontier, but I use Thunderbird. I hope that works.

My setting was delete after 14 days, but that has always been the same. I've only had the problem since the last update. I've changed it to Never.