iOS 16.4 update problems with email account

Everything that you are telling us indicates that your mail provider has not yet updated their system for iOS 16.4. Contact the support folks at your mail provider to ask them when they expect to have an update implemented on their systems.

ABreuder wrote:

I just updated to the current iPhone OS over the weekend. Ever since then, I'm not getting my regular email due to an error with the email account password/username. I've checked my account details and everything is correct. If I shut down the phone, then re-start it, all my messages for that account load correctly. Any subsequent attempt to update the message list immediately results in the account username/password error. This was not an issue prior to this update.

contact your mail provider or look at their support site.