send_mail.rb

require "google/apis/gmail_v1"
require "googleauth"
require "googleauth/stores/file_token_store"
require "fileutils"
require "mail"
require "pry"

OOB_URI = "urn:ietf:wg:oauth:2.0:oob".freeze
APPLICATION_NAME = "Gmail API Ruby Mail Sender".freeze
CREDENTIALS_PATH = "credentials.json".freeze
# The file token.yaml stores the user's access and refresh tokens, and is
# created automatically when the authorization flow completes for the first
# time.
TOKEN_PATH = "token.yaml".freeze
SCOPE = Google::Apis::GmailV1::AUTH_GMAIL_SEND # grant access to send

##
# Ensure valid credentials, either by restoring from the saved credentials
# files or intitiating an OAuth2 authorization. If authorization is required,
# the user's default browser will be launched to approve the request.
#
# @return [Google::Auth::UserRefreshCredentials] OAuth2 credentials
def authorize
  client_id = Google::Auth::ClientId.from_file CREDENTIALS_PATH
  token_store = Google::Auth::Stores::FileTokenStore.new file: TOKEN_PATH
  authorizer = Google::Auth::UserAuthorizer.new client_id, SCOPE, token_store
  user_id = "default"
  credentials = authorizer.get_credentials user_id
  if credentials.nil?
    url = authorizer.get_authorization_url base_url: OOB_URI
    puts "Open the following URL in the browser and enter the " \
         "resulting code after authorization:"
    puts url
    puts "Enter the authorization code:"
    code = gets
    credentials = authorizer.get_and_store_credentials_from_code(
      user_id: user_id, code: code, base_url: OOB_URI
    )
  end
  credentials
end

# Initialize the API
service = Google::Apis::GmailV1::GmailService.new
service.client_options.application_name = APPLICATION_NAME
service.authorization = authorize

# create message
mail = Mail.new do
  to "MAIL_ID_TO_WHOM_YOU_WANT_TO_SEND"
  from "YOUR_MAIL_ID"
  subject "Hello from ruby"
  html_part do
    content_type "text/html; charset=UTF-8"
    body "This e-mail is sent from Gmail API via ruby. Isn't that Cool?"
  end
end

service.send_user_message(
  "me", upload_source: StringIO.new(mail.to_s),
  content_type: "message/rfc822"
)
puts "Message Id: #{mail.message_id}"