Hi,
I followed this video [1] where they configure the Dialogflow to send emails using Nodemailer [2].
At first sight, it seems like, in case you are not missing any of the components detailed in the video, the issue should be related to the security of the Gmail app. Make sure that you enable the less secure apps in [3] to use Gmail to send the emails.
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[1] -
https://www.youtube.com/watch?v=ErVYtGV43R8
[2] -
[3] -
https://www.google.com/settings/security/lesssecureapps