I am having this problem on my iphone 7 and I have Sprint. I really don't want to switch to IMAP so I have been dealing with it. I can usualy get mail one time after I'm off WIFI and then it doesn't work again until I get back on WIFI. It doesn't sound like IMAP fixes the problem anyway? Have you found anything that fixes this?
I have been having this problem too (cannot access Verizon POP3 email over Consumer Cellular, although it works over WiFi). I last tried this weeks ago. I tried it again today, and it now seems to work properly. Hardware was iPhone 6.
Before this fix, I was able to get Verizon email by first launching a VPN before opening the Mail app. That should no longer be necessary though.
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