input fields – Best practices for validating emails in a text box

I want to allow my users to add recipients to a text address in a text box by simply writing their address manually or by copying and pasting them somewhere. Email addresses are separated by a comma or a new line.

How to validate an email address?

If it is an incorrect address, say,, how do you tell the user that it is incorrect?

If it's an incorrect address, say, john.doe @ example, com, how do you tell the user that this is incorrect?

A) Show them a notification in the text box?

If you have several errors, you can always expand the notification below and scroll through it.
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B) Offer a tip / notification?

This option would be very frustrating when the user makes several mistakes.
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C) Do you offer a validation?

As I understand it, the validation of e-mails is not 100% accurate, whatever you do – you can never have any critical cases thought. You may create a nuisance if you do not validate a legitimate email.