FIX: improve mailman email parsing (#21627)

https://meta.discourse.org/t/improving-mailman-email-parsing/253041

When mirroring a public mailling list which uses mailman, there were some cases where the incoming email was not associated to the proper user.

As it happens, for various (undertermined) reasons, the email from the sender is often not in the `From` header but can be in any of the following headers: `Reply-To`, `CC`, `X-Original-From`, `X-MailFrom`.

It might be in other headers as well, but those were the ones we found the most reliable.

spec/lib/email/receiver_spec.rb

@@ -218,14 +218,6 @@ RSpec.describe Email::Receiver do
     expect(IncomingEmail.last.error).to eq("RuntimeError")
   end
 
-  it "matches the correct user" do
-    user = Fabricate(:user)
-    email_log = Fabricate(:email_log, to_address: user.email, user: user, bounce_key: nil)
-    email, name = Email::Receiver.new(email(:existing_user)).parse_from_field
-    expect(email).to eq("existing@bar.com")
-    expect(name).to eq("Foo Bar")
-  end
-
   it "strips null bytes from the subject" do
     expect do process(:null_byte_in_subject) end.to raise_error(
       Email::Receiver::BadDestinationAddress,
@@ -512,9 +504,8 @@ RSpec.describe Email::Receiver do
       )
     end
 
-    it "handles invalid from header" do
-      expect { process(:invalid_from_1) }.to change { topic.posts.count }
-      expect(topic.posts.last.raw).to eq("This email was sent with an invalid from header field.")
+    it "raises a NoSenderDetectedError when the From header can't be parsed" do
+      expect { process(:invalid_from_1) }.to raise_error(Email::Receiver::NoSenderDetectedError)
     end
 
     it "raises a NoSenderDetectedError when the From header doesn't contain an email address" do
@@ -1996,6 +1987,38 @@ RSpec.describe Email::Receiver do
     end
   end
 
+  describe "mailman mirror" do
+    fab!(:category) { Fabricate(:mailinglist_mirror_category) }
+
+    it "uses 'from' email address" do
+      expect { process(:mailman_1) }.to change { Topic.count }
+      user = Topic.last.user
+      expect(user.email).to eq("some@one.com")
+      expect(user.name).to eq("Some One")
+    end
+
+    it "uses 'reply-to' email address" do
+      expect { process(:mailman_2) }.to change { Topic.count }
+      user = Topic.last.user
+      expect(user.email).to eq("some@one.com")
+      expect(user.name).to eq("Some")
+    end
+
+    it "uses 'x-mailfrom' email address and name from CC" do
+      expect { process(:mailman_3) }.to change { Topic.count }
+      user = Topic.last.user
+      expect(user.email).to eq("some@one.com")
+      expect(user.name).to eq("Some One")
+    end
+
+    it "uses 'x-original-from' email address" do
+      expect { process(:mailman_4) }.to change { Topic.count }
+      user = Topic.last.user
+      expect(user.email).to eq("some@one.com")
+      expect(user.name).to eq("Some")
+    end
+  end
+
   describe "mailing list mirror" do
     fab!(:category) { Fabricate(:mailinglist_mirror_category) }