2007931 - Thunderbird inserts >From in message header, thus, message has invalid header

Status: UNCONFIRMED

(Thunderbird :: Untriaged, defect)

Description

[Roy Planalp]

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/143.0.0.0 Safari/537.36

Steps to reproduce:

no idea what component
I viewed the source of my messages in my Exchange Server account (via Owl) Inbox, and find the first line of the message source starts ">From.."
for example, some lines of the header on a very old message, trying to bold them

>From - Fri Nov 14 2025 11:22:14
X-Mozilla-Status: 0001
X-Mozilla-Status2: 00000000
Received: from MN2PR04MB6479.namprd04.prod.outlook.com (2603:10b6:208:2d::14)
by MN2PR04MB6271.namprd04.prod.outlook.com with HTTPS via
BL0PR03CA0001.NAMPRD03.PROD.OUTLOOK.COM; Mon, 27 Apr 2020 18:21:52 +0000
Received: from MN2PR04MB6960.namprd04.prod.outlook.com (2603:10b6:208:1e7::21)
by MN2PR04MB6479.namprd04.prod.outlook.com (2603:10b6:208:1a2::8) with
Microsoft SMTP Server (version=TLS1_2,
cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2937.13; Mon, 27 Apr
2020 18:21:52 +0000
Received: from MN2PR04MB6960.namprd04.prod.outlook.com
([fe80::d8e6:c0fe:d437:dc89]) by MN2PR04MB6960.namprd04.prod.outlook.com
([fe80::d8e6:c0fe:d437:dc89%5]) with mapi id 15.20.2937.023; Mon, 27 Apr 2020
18:21:52 +0000
From: "Varga, Krisztina" <[e-mail]>
To: "Planalp, Roy" <[e-mail]>
Subject: UNH URC Interdisciplinary Science & Engineering Symposium (ISE)
Biology presentations on Wednesday, April 29th

This is true without regard for the actual date of the message.
→ It is as though some processing of my Inbox has happened to insert this bogus header in front of every message.
I am aware that the mbox format includes a From header to delimit the start of messages. And further that in the message text, >From is done on a line starting with From. But this > is not in the message text!
**→ this is preventing me transferring the message to Proton's IMAP server, as a way to archive messages - the messages are rejected "Illegal header character" - which is true.
This bad header is not at the start of outlook messages

Actual results:

Messages have an invalid header

Expected results:

The

From xxx
line should not appear at all in the message.