例えば UserMailer でキューを指定する方法。

class UserMailer < ApplicationMailer
  ...
end

deliver_later で指定

UserMailer.with(user:).welcome_email.deliver_later(queue: :high_priority)

see also. ActionMailer::MessageDelivery | RailsDoc(β)

delivery_job で指定

class UserMailer < ApplicationMailer
  self.delivery_job = MyDeliveryJob
end
class MyDeliveryJob < ApplicationJob
  queue_as :high_priority
end

参考資料