Emails won't send and are undeliverable

Are you using a 3rd party mail app or the default Apple Mail app?

Are you sending mail via your Apple email address or a 3rd part email account address?

Choose the correct email provider when adding an account to Mail - Apple Support

Any error messages given after sending? Does the email come back with an attached error message from the provider?

Did you already have a look at the troubleshooting steps mentioned in the related article from Apple?

If you can’t send email on your iPhone or iPad - Apple Support