Linking email accounts

You may have enabled 2-step verification for the email service provider. In that case, you need to generate an App-specific password for the iOS mail app. Sign in to apps with your Apple ID using app-specific passwords - Apple Support (IN)

Google

An App Password is a 16-digit passcode that gives a non-Google app or device permission to access your Google Account. App Passwords can only be used ... Sign in using App Passwords - Google Account Help

Yahoo

Watch the GIF below for generating and using app-specific Yahoo password on iOS mail app. On other Yahoo app and on a web browser you do not require to generate this app specific password --> Generate and manage third-party app passwords

GIF for app specific password

Outlook

Have you enabled two-step verification with your Microsoft account (Outlook)

You will have to generate app specific password --> Using app passwords with apps that don't support two-step ...

You have not mentioned which email account is that.

You may have enabled two step verification on Google --> Sign in with App Passwords - Google Account Help

You may have enabled Authenticator for Yahoo --> Generate and manage third-party app passwords | Yahoo Help

You may have enbled 2-step verification for Outlook --> Manage app passwords for two-step verification

You may have activate 2-step verification for Aol --> Create and manage third-party app passwords - AOL Help