Deliver_later not working for legacy site

Question

I've got a legacy site I'm working on that is giving me a new error. It's Rails 5.1.0.rc2.

Site uses the below similar coding in other mailer actions, which are giving this error:

Error performing ActionMailer::DeliveryJob (Job ID: 03897c07-3397-4b37-8c62-cfcf12dd1905) from Async(mailers) in 244.32ms: NoMethodError (undefined method `deliver_later' for #<Mail::Message:0x007fe7e5c9d9c8>
Did you mean?  delivery_handler):
/home/dave/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/mail-2.7.0/lib/mail/message.rb:1396:in `method_missing'

controller:

UserMailer.hpe_new_vehicle_applicant(@dealer,@vehicle,@user).deliver_later

user_mailer:

def hpe_new_vehicle_applicant(dealer,vehicle,user)
    @dealer = dealer
    @vehicle = vehicle
    @user = user

    account_emails = @dealer.sales_persons.where.not(confirmed_at: nil).pluck(:email)
    subject = "no reply - New Vehicle Applicant (#{@dealer.dealer_name})"

    account_emails.each do |email|
      send_user_mail(email, subject).deliver_later
    end

  end

I'm thinking the code worked previously, but I'm not sure. Also no changes have been done except to update bcrypt gem. ... I'm also not seeing why send_user_mail is being used. No real results show when googled.

Update: 'send_user_mail(email, subject)' was a call to a function within user_mailer that sent the email, which at some point acquired .deliver_later that broke it.