class WoodlockWelcomeMailer < ApplicationMailer
  def omniauth_welcome(user, provider)
    case provider
    when 'google_oauth2' then provider_name = 'Google'
    when 'facebook' then provider_name = 'Facebook'
    else provider_name = 'unknown provider'
    end

    @greeting = "Hi #{user.first_name}! Thanks for your registration."

    mail to: user.email, subject: "#{ Woodlock.site_name } - #{ provider_name } registration success"
  end
end

class WoodlockWelcomeMailer < ApplicationMailer
  def omniauth_welcome(user, provider)
    case provider
    when 'google_oauth2' then provider_name = 'Google'
    when 'facebook' then provider_name = 'Facebook'
    else provider_name = 'unknown provider'
    end

    @greeting = "Hi #{user.first_name}! Thanks for your registration."

    mail to: user.email, reply_to: Woodlock.site_email, subject: "#{ Woodlock.site_name } - #{ provider_name } registration success"
  end
end

Add reply_to inside mail class.
Add reply_to inside mail class.
