SStructured testing over the weekend has shown that this problem is also potentially expensive when connecting over cellular.
What appears to be happening, for all email, is that the iOS mail-client is attempting to unconditionally reload all "embedded" content from the mail-server (over the active network connection) at every attempt to read the email Message. If you monitor the network traffic when accessing a message, unless the local email message content is simple text only, you'll see that the mail client will attempt to download the message again from the server. If the network connection is missing, you'll not be able to view the message - despite having previously downloaded its content.
THis repetitive downloading of messages will add considerably to your cellular data usage. Whilst repetitive loading of message content over WiFi has a lower cost, regardless of the network bearer in use, you can only "read" an email if you have an active connection to the mail server. As is, this makes the embedded iOS mail client no better than web-mail for reading any received messages.
THis expensive, repetitive and unnecessary download of graphical content can, in part, be mitigated by changing a mail-setting (disable "load remote images" in mail settings). This prevents downloading/re-downloading of graphical message content on every access, but does not resolve the underlying fault.
When, if, Apple ever get around to fixing this maddening fault, perhaps they'll fix the double-capitalisation issue that again affects text input from the virtual keyboard (of which this response shows several examples). Come on Apple, time to rapidly fix these problems instead of just concentrating on your profits; we, the customer, have all contributed to your bottom-line.
