Mail

open system preferences, Mail/Accounts/your email-account...

This should bring you to the apple-id panel. Usually, the mail start to work as soon as you are correctly logged in.

Also, have a look at this:

https://support.apple.com/en-us/HT201419

https://support.apple.com/en-us/HT202304

hth

david

It sometimes happens on my iPhone, and all I had to do (if it didn't fix itself) was to reconnect my apple-id...

the first link also treats those problems on iPads I thought?

Sorry if it didn't help...