Hi J,
Thanks for reaching out through our Apple Support Communities for help with this issue loading your email messages on iPhone after the latest update. Rest assured, we're glad to look into this with you and help in any way we can.
To start, let's ensure that the Mail app has been forced to close before testing again. Here's a guide with the steps to force an app to close if it's not working as expected:
How to force an app to close on your iPhone, iPad, or iPod touch
If you find that the same behavior is still occurring upon reopening Mail, we recommend restarting your iPhone next. This guide explains the process for a standard restart:
Restart your iPhone
Restarting helps to refresh your iPhone's apps and memory, which may resolve unexpected software behavior. If you find that the same issue is still occurring, we'll want to take steps to isolate the root cause further.
Will it be possible to test while joined to a different Wi-Fi network, or after switching to cellular data?
If you find that the same thing is happening regardless of your network connection, it may indicate an issue with the configuration of your Mail account or accounts in the Settings app.
Is this issue affecting multiple different Mail accounts? You'll want to head into Settings > Passwords & Accounts, and then verify that any Mail accounts you've configured are signed in without issue.
The following guide has some great steps to help address issues in individual Mail accounts. Let us know if this helps, or if you've got any questions:
If you can’t send email on your iPhone, iPad, or iPod touch
Have a great day!