adding a new email account on mac book

Whatever you're entering for your account is incorrect. There's a username and a password for the reading and receiving server (POP or IMAP) path, and a second set of credentials and settings for the sending server (SMTP or ESMTP) path. Both sets of credentials and all of the server-related settings must be correct.

With iCloud Keychain enabled, the mail server settings are shared across all devices and Macs sharing that Apple ID.

I’ll assume you’re using the other service when trying to configure the new account.

Is iCloud Keychain enabled here?

Here are the necessary settings:

https://www.ionos.com/help/email/other-email-programs/set-up-an-email-account-in-apple-mail/

here is what IONOS recommends:

https://www.ionos.com/help/email/other-email-programs/set-up-an-email-account-in-apple-mail/

Some services can be case-sensitive.

Stay away from the unencrypted ports; do not use 143, and 110.

Using Other, or one of the existing mail account profiles?

Which mail provider?