I want to allow my users to add email recipients in a text area by just writing their addresses manually or copy & paste them from somewhere - email addresses are separated via a comma or new line.

How do you validate an email address?

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

A) Show them a notification under the text area?

If you've got multiple mistakes, you can always expand the notification underneath and make it scrollable.

B) Offer a tooltip / notification?

This is option would be very frustrating when the user has multiple mistakes.

C) Do you offer any validation at all?

AFAIK, email validation isn't 100% accurate whatever you do - you can never have all the edge cases thought through. So, you might end up creating a nuisance if you don't validate a legitimate email.