I have an iPhone 6s running iOS 11.3.1. The Mail app is able to download and display e-mail from my GMail account. However, e-mail to my account at register.com is frequently not displayed until I have powered off the phone and powered it back on. The webmail service at register.com shows that the e-mails have been received, and the iPhone actually shows a little message saying "Downloading 5 of 5" or whatever, but the e-mails never appear.
We all know that 11.3.1 is something of a failed release, but has anyone else experienced this problem and found a solution that doesn't involve turning the phone off and on to receive new mail?