Answer by Patrick Allaert for How to send an email, using google gmail api in php with cURL?

Looks like you are sending JSON encoded data while you should respect message/rfc822 format.

You should probably not base64-encode + json-encode your message:

<?php$message = "To: test@example.com\r\nFrom: test@example.com\r\nSubject: GMail test.\r\n My message";$ch = curl_init('https://www.googleapis.com/upload/gmail/v1/users/me/messages/send'); curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);curl_setopt($ch, CURLOPT_POST, 1);curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);curl_setopt($ch, CURLOPT_HTTPHEADER, array("Authorization: Bearer $AccessToken", 'Accept: application/json', 'Content-Type: message/rfc822'));    curl_setopt($ch, CURLOPT_POSTFIELDS, $message);$data = curl_exec($ch);