UX: better rejection message when reply via email is too short

config/locales/server.en.yml

@@ -2502,6 +2502,14 @@ en:
 
         If you can correct the problem, please try again.
 
+    email_reject_post_too_short:
+      title: "Email Reject Post Too Short"
+      subject_template: "[%{email_prefix}] Email issue -- Post too short"
+      text_body_template: |
+        We're sorry, but your email message to %{destination} (titled %{former_title}) didn't work.
+
+        To promote more in depth conversations, very short replies are not allowed. Can you please reply with at least %{count} characters? Alternatively, you can like a post via email by replying with "+1".
+
     email_reject_invalid_post_action:
       title: "Email Reject Invalid Post Action"
       subject_template: "[%{email_prefix}] Email issue -- Invalid Post Action"

lib/email/processor.rb

@@ -52,6 +52,7 @@ module Email
                          when Email::Receiver::TopicNotFoundError          then :email_reject_topic_not_found
                          when Email::Receiver::TopicClosedError            then :email_reject_topic_closed
                          when Email::Receiver::InvalidPost                 then :email_reject_invalid_post
+                         when Email::Receiver::TooShortPost                then :email_reject_post_too_short
                          when Email::Receiver::UnsubscribeNotAllowed       then :email_reject_invalid_post
                          when ActiveRecord::Rollback                       then :email_reject_invalid_post
                          when Email::Receiver::InvalidPostAction           then :email_reject_invalid_post_action
@@ -69,6 +70,10 @@ module Email
         template_args[:post_error] = e.message
       end
 
+      if message_template == :email_reject_post_too_short
+        template_args[:count] = SiteSetting.min_post_length
+      end
+
       if message_template == :email_reject_unrecognized_error
         msg  = "Unrecognized error type (#{e.class}: #{e.message}) when processing incoming email"
         msg += "\n\nBacktrace:\n#{e.backtrace.map { |l| "  #{l}" }.join("\n")}"

lib/email/receiver.rb

@@ -31,6 +31,7 @@ module Email
     class TopicNotFoundError           < ProcessingError; end
     class TopicClosedError             < ProcessingError; end
     class InvalidPost                  < ProcessingError; end
+    class TooShortPost                 < ProcessingError; end
     class InvalidPostAction            < ProcessingError; end
     class UnsubscribeNotAllowed        < ProcessingError; end
     class EmailNotAllowed              < ProcessingError; end
@@ -932,7 +933,14 @@ module Email
       user = options.delete(:user)
       result = NewPostManager.new(user, options).perform
 
-      raise InvalidPost, result.errors.full_messages.join("\n") if result.errors.any?
+      errors = result.errors.full_messages
+      if errors.any? do |message|
+           message.include?(I18n.t("activerecord.attributes.post.raw").strip) &&
+           message.include?(I18n.t("errors.messages.too_short", count: SiteSetting.min_post_length).strip)
+         end
+        raise TooShortPost
+      end
+      raise InvalidPost, errors.join("\n") if result.errors.any?
 
       if result.post
         @incoming_email.update_columns(topic_id: result.post.topic_id, post_id: result.post.id)

spec/components/email/processor_spec.rb

@@ -5,6 +5,52 @@ describe Email::Processor do
 
   let(:from) { "foo@bar.com" }
 
+  context "when reply via email is too short" do
+    let(:mail) { file_from_fixtures("email_reply_3.eml", "emails").read }
+    let(:post) { Fabricate(:post) }
+    let!(:user) { Fabricate(:user, email: "three@foo.com") }
+    let!(:email_log) do
+      Fabricate(:email_log,
+        message_id: "35@foo.bar.mail", # don't change, based on fixture file "email_reply_3.eml"
+        email_type: "user_posted",
+        post_id: post.id,
+        to_address: "asdas@dasdfd.com"
+      )
+    end
+
+    before do
+      SiteSetting.min_post_length = 1000
+      SiteSetting.find_related_post_with_key = false
+    end
+
+    it "rejects reply and sends an email with custom error message" do
+      begin
+        receiver = Email::Receiver.new(mail)
+        receiver.process!
+      rescue Email::Receiver::TooShortPost => e
+        error = e
+      end
+
+      expect(error.class).to eq(Email::Receiver::TooShortPost)
+      processor = Email::Processor.new(mail)
+      processor.process!
+
+      rejection_raw = processor.send(:handle_failure, mail, error).body.raw_source
+
+      count = SiteSetting.min_post_length
+      destination = receiver.mail.to
+      former_title = receiver.mail.subject
+
+      expect(rejection_raw.gsub(/\r/, "")).to eq(
+        I18n.t("system_messages.email_reject_post_too_short.text_body_template",
+          count: count,
+          destination: destination,
+          former_title: former_title
+        ).gsub(/\r/, "")
+      )
+    end
+  end
+
   describe "rate limits" do
 
     let(:mail) { "From: #{from}\nTo: bar@foo.com\nSubject: FOO BAR\n\nFoo foo bar bar?" }

spec/components/email/receiver_spec.rb

@@ -207,7 +207,7 @@ describe Email::Receiver do
     end
 
     it "raises an InvalidPost when there was an error while creating the post" do
-      expect { process(:too_small) }.to raise_error(Email::Receiver::InvalidPost)
+      expect { process(:too_small) }.to raise_error(Email::Receiver::TooShortPost)
     end
 
     it "raises an InvalidPost when there are too may mentions" do