Submitting many ActionMail jobs at once with Sidekiq

I recently came across a situation where enqueueing several thousand mails from a Rails app to Sidekiq was surprisingly slow and the fix surprisingly complex, so I decided to write a blog post about it both for myself (if I need this again) and others (if they have the same problem).The core problem starts when you have a bunch of mails to send that are similar but slightly different. In this case, this was because we wanted to include the name of the user (ie "Hello, .