Send email notification to node author with Rules I am trying to notify the node author of a new comment on their node. 

Using Drupal 8.3

Created new reaction rule. 

Send to:  comment.entity_id.entity.uid.entity.mail.value

email address is not being picked up

I get this email from system: 

A message that you sent contained no recipient addresses, and therefore no
delivery could be attempted.
This worked. 

As a workaround, you can use the data selection string as a token in the direct input textarea.

Switch to data selection and get your string, in my case content_moderation_state.uid.entity.mail.value

Switch to the direct input mode and paste the string with double braces {{content_moderation_state.uid.entity.mail.value}} to use the token replacement API.

As described here: https://www.drupal.org/project/rules/issues/2799569#comment-12376804