send_email matcher
The send_email matcher is used to check if an email with the given parameters has been sent inside the expectation block.
NOTE: This matcher implies that the example(s) actually send email(s) using the test adapter, and not scheduled as a job to be sent in the background later.
To have an email sent in tests make sure:
  - ActionMailer performs deliveries - Rails.application.config.action_mailer.perform_deliveries = true
  - If the email is sent asynchronously (with .deliver_later call), ActiveJob uses the inline adapter - Rails.application.config.active_job.queue_adapter = :inline
  - ActionMailer uses the test adapter - Rails.application.config.action_mailer.delivery_method = :test
If you want to check an email has been scheduled as a job, use the have_enqueued_email matcher.
Checking email sent with the given multiple parameters
Given a file named “spec/mailers/notificationsmailerspec.rb” with:
require "rails_helper"
RSpec.describe NotificationsMailer do
  it "checks email sending by multiple params" do
    expect {
      NotificationsMailer.signup.deliver_now
    }.to send_email(
      from: 'from@example.com',
      to: 'to@example.org',
      subject: 'Signup'
    )
  end
end
When I run rspec spec/mailers/notifications_mailer_spec.rb
Then the examples should all pass.
Checking email sent with matching parameters
Given a file named “spec/mailers/notificationsmailerspec.rb” with:
require "rails_helper"
RSpec.describe NotificationsMailer do
  it "checks email sending by one param only" do
    expect {
      NotificationsMailer.signup.deliver_now
    }.to send_email(
      to: 'to@example.org'
    )
  end
end
When I run rspec spec/mailers/notifications_mailer_spec.rb
Then the examples should all pass.
Checking email not sent with the given parameters
Given a file named “spec/mailers/notificationsmailerspec.rb” with:
require "rails_helper"
RSpec.describe NotificationsMailer do
  it "checks email not sent" do
    expect {
      NotificationsMailer.signup.deliver_now
    }.to_not send_email(
      to: 'no@example.org'
    )
  end
end
When I run rspec spec/mailers/notifications_mailer_spec.rb
Then the examples should all pass.