无法在 iPhone 和 iPad 上使用电子邮件

In the case where you have previously added and used email on your iPad or iPhone normally, then suddenly it doesn't work. (I've encountered this problem with iOS version 10 onwards.) To solve the problem of not being able to use email in cases like this, fix it as follows:

Check the SMTP section that is not the main one. If there are SMTP values other than the main one, delete them all. After that, delete the account that is being used and then add it again (in the case that there is mail on the server, after adding it again, the system will download the email again)