[macanas]

When someone answer a post, tell the creator with a email.

Which function has to change?

[chrishajer]

Email notifications are built in to the latest trunk version. You can “subscribe to topic” here, for example, and you will get an email when there is a reply to your post. It requires the manual step of you actually subscribing. There were also plugins to enable this in older versions, to email you when someone replied to a favorite of yours.