fix: unknown charset windows-874 problem on incoming mail

When the sender is using email with windows-874 charset (i.e., Outlook / Thai), the incoming email, i.e., to Issue will result in weird characters.

This is due to, python don't know about this charset. This fix by using alias charset for the problematic charset.

frappe/email/receive.py

@@ -3,6 +3,7 @@
 
 import datetime
 import email
+import email.charset
 import email.utils
 import imaplib
 import json
@@ -38,6 +39,13 @@
 from frappe.utils.html_utils import clean_email_html
 from frappe.utils.user import is_system_user
 
+# use alias charset for python unknown charset
+email.charset.ALIASES.update(
+	{
+		"windows-874": "cp874",
+	}
+)
+
 # fix due to a python bug in poplib that limits it to 2048
 poplib._MAXLINE = 1_00_000
 
@@ -405,10 +413,12 @@ def set_subject(self):
 		"""Parse and decode `Subject` header."""
 		_subject = decode_header(self.mail.get("Subject", "No Subject"))
 		self.subject = _subject[0][0] or ""
+		charset = _subject[0][1]
 
-		if _subject[0][1]:
+		if charset:
 			# Encoding is known by decode_header (might also be unknown-8bit)
-			self.subject = safe_decode(self.subject, _subject[0][1])
+			charset = email.charset.ALIASES.get(charset, charset)
+			self.subject = safe_decode(self.subject, charset)
 
 		if isinstance(self.subject, bytes):
 			# Fall back to utf-8 if the charset is unknown or decoding fails
@@ -502,7 +512,7 @@ def get_charset(self, part):
 
 	def get_payload(self, part):
 		charset = self.get_charset(part)
-
+		charset = email.charset.ALIASES.get(charset, charset)
 		try:
 			return str(part.get_payload(decode=True), str(charset), "ignore")
 		except LookupError: