iPad Air with iOS 8.3 arbitrarily deleting e-mails

Whilst I have used Macs for 30 years, I'm fairly new at using an iPad though I have had a couple of iPhones which I have used to check my e-mail in certain occasional situations.

I got my iPad Air just before Christmas and most Mail settings were left at default. I have two POP3 e-mail accounts which I manage manually online, so I delete messages on the server myself - a habit I have maintained since having a serious spam problem a few years ago (now resolved).

I noticed a couple of days ago that an e-mail to which I wanted to refer from around February was no longer there and then realised that there was no mail over a month old on my iPad. I set about finding out why it all might have vanished and discovered the Advanced Mail settings which set the deletion time for e-mail which I then set to never.

I wasn't sure whether my mail might then miraculously reappear or if the setting would only be effective from now on but after a sync with my Mac, I found that although there was e-mail dating back to December, some messages, notably from people flagged as VIPs were now gone right up to messages received this week. In fact, of seven VIPs, every single message from five of them has gone and the messages from the other two only go back a few days.

I know that since I got my iPad, every e-mail I have received on my server has arrived on my iPad and my Mac before it has been deleted from the server but I don't know how the messages are managed on the iPad. It certainly doesn't make sense that some mail from almost five months ago is now in my inbox yet mail from VIPs to the same account are gone. I have not deleted any messages from my iPad at all other than marking the occasional message as spam - there are no messages in the Trash folder of either account.

I have suddenly lost faith in my iPad as a reliable primary communication tool and feel that I can only use it as a secondary device. I'm very disappointed.