I agree with carolinseattle on this. There is no way to be always connected via WiFi, and we don't all have large data plans. I don't use much data so I would only be squandering my money on increasing it again. In fact, I did up my data plan, just before a trip in July, as I was sure I'd need it for Google maps usage, which I did. When I was trying to read the email with my airline confirmation number on it, so I could check-in online using my iPhone, I was at a wedding reception in downtown Milwaukee the first weekend in October. I turned on my cellular data, after not being able to see the reservation and confirmation info using just my normal T-Mobile connection, as I could when I was connected via WiFi, but the email still didn't show me the body of the message, which, btw, was not an attachment. I could only see the paperclip icon and upon opening the email, got the message... not yet downloaded from the server. As I had previously been able to read it on my phone while connected to WiFi, several times in fact because I made sure I could see it all before leaving the hotel, it should have been downloaded to my iPhone and stayed there, but alas, this wasn't the case.
All my travel emails are on my @icloud.com or @me.com account, they and all my other email accounts are all IMAP, and the problem is there for me with all of them. I don't have any POP or POP3 email accounts, so it's not limited to POP accounts.
I'm wondering if it has anything to do with how much memory is being allotted to email on our iPhones, yet there is no way to increase this, even if we have a ton of open memory, as I do. I don't keep much on my iPhone in terms of iTunes, photos, etc. so for me, there shouldn't be any reason why this is happening. Some say yes, but upon deleting everything on their iPhones or iPads and resetting them to new, the problem persists.
😠 Apple is STILL in the doghouse!!!