From 8a080fb0764e2b882eb51a3237a653f918950055 Mon Sep 17 00:00:00 2001
From: Sami Samhuri <sami@samhuri.net>
Date: Tue, 28 Dec 2021 15:22:49 -0800
Subject: [PATCH] Perform >From quoting as described by RFC 4155

https://www.rfc-editor.org/rfc/rfc4155.txt
http://qmail.org/qmail-manual-html/man5/mbox.html

> The program then copies the message, applying >From quoting
> to each line.  >From quoting ensures that the resulting
> lines are not From_ lines:  the program prepends a > to any
> From_ line, >From_ line, >>From_ line, >>>From_ line, etc.
---
 imapbackup.py   | 6 ++++++
 imapbackup38.py | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/imapbackup.py b/imapbackup.py
index 4a0f0fe..8325407 100644
--- a/imapbackup.py
+++ b/imapbackup.py
@@ -167,6 +167,7 @@ def download_messages(server, filename, messages, config):
     spinner = Spinner("Downloading %s new messages to %s" % (len(messages), filename),
                       config['nospinner'])
     total = biggest = 0
+    from_re = re.compile("\n(>*)From ")
 
     # each new message
     for msg_id in messages.keys():
@@ -190,6 +191,11 @@ def download_messages(server, filename, messages, config):
             # This avoids Thunderbird mistaking a line starting "From  " as the start
             # of a new message. _Might_ also apply to other mail lients - unknown
             text = text.replace("\nFrom ", "\n From ")
+        else:
+            # Perform >From quoting as described by RFC 4155 and the qmail docs.
+            # https://www.rfc-editor.org/rfc/rfc4155.txt
+            # http://qmail.org/qmail-manual-html/man5/mbox.html
+            text = from_re.sub("\n>\\1From ", text)
         mbox.write(text)
         mbox.write('\n\n')
 
diff --git a/imapbackup38.py b/imapbackup38.py
index bb594f4..3c79611 100644
--- a/imapbackup38.py
+++ b/imapbackup38.py
@@ -155,6 +155,7 @@ def download_messages(server, filename, messages, overwrite, nospinner, thunderb
     spinner = Spinner("Downloading %s new messages to %s" % (len(messages), filename),
                       nospinner)
     total = biggest = 0
+    from_re = re.compile(b"\n(>*)From ")
 
     # each new message
     for msg_id in messages.keys():
@@ -183,6 +184,11 @@ def download_messages(server, filename, messages, overwrite, nospinner, thunderb
             # This avoids Thunderbird mistaking a line starting "From  " as the start
             # of a new message. _Might_ also apply to other mail lients - unknown
             text_bytes = text_bytes.replace(b"\nFrom ", b"\n From ")
+        else:
+            # Perform >From quoting as described by RFC 4155 and the qmail docs.
+            # https://www.rfc-editor.org/rfc/rfc4155.txt
+            # http://qmail.org/qmail-manual-html/man5/mbox.html
+            text_bytes = from_re.sub(b"\n>\\1From ", text_bytes)
         mbox.write(text_bytes)
         mbox.write(b'\n\n')