Attached is a mailfile (set rail-mode) with one message. Typing "r" to reply I see ------------------------------------------------------------------ From: jpff To: Steven Yi CC: Victor.Lazzarini@mu.ie In-reply-to: (message from Steven Yi on Tue, 19 Sep 2017 08:39:00 -0400) Subject: Re: Next release BCC: jpff@codemist.co.uk References: <9c571770-2e28-4b78-95c6-baad81925043@codemist.co.uk> <63268264-9DE9-4EDE-AE92-BC3236D5A1FF@mu.ie> --text follows this line-- ==John ffitch ------------------------------------------------------------------------ (that is my automatically inserted signature) Adding text `"test" to this and sending the message I get (my BCC) is ------------------------------------------------------------------------ Date: Tue, 19 Sep 2017 21:13:14 +0100 From: jpff To: Steven Yi Cc: "Victor.Lazzarini@mu.ie" Subject: Re: Next release test ==John ffitch ------------------------------------------------------------------------ which is clearly ill formed -- and indeed I also get the message ------------------------------------------------------------------------ Date: Tue, 19 Sep 2017 21:13:51 +0100 From: Mail Delivery System To: jpff@codemist.co.uk Subject: Mail delivery failed: returning message to sender Parts/Attachments: 1 Shown 8 lines Text 2 Shown 161 bytes Message, "Delivery Status" 3 Shown 1.2 KB Message, "Re: Next release" 3.1 Shown 3 lines Text ---------------------------------------- This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: CANtcCs7-TwLw9PiOvQBYyertQbWurCsJ-i9B9JjGQTZxoaNesg@mail.gmail.com Unknown user [ Part 2: "Delivery Status" ] Reporting-MTA: dns; codemist.co.uk Action: failed Final-Recipient: rfc822;CANtcCs7-TwLw9PiOvQBYyertQbWurCsJ-i9B9JjGQTZxoaNesg@mail.gmail.com Status: 5.0.0 [ Part 3: "Included Message" ] Date: Tue, 19 Sep 2017 21:13:14 +0100 From: jpff To: Steven Yi Cc: "Victor.Lazzarini@mu.ie" Subject: Re: Next release test ==John ffitch ------------------------------------------------------------------------ Experiments suggest that if I move the I-reply-to lines to the end of the headers it works Hope that is clear? ==John ffitch