I just did the same 3 tests using my Fastmail, and the results were the same/sane as Bradford's results below.

That is to be expected, because the two chief designers at Fastmail are also two of the chief architects of the JMAP initiative, an IETF standard intended to ultimately replace IMAP.

Hartmut

On Sat 18 Apr 2026 at 07:52:58 -05:00, Bradford Vokey via Roundtable <roundtable@muug.ca> wrote:

- Reply_to_Sender_Only [Ctrl-R]: replies to the "Reply-To" header (Trevor). Sane.
- Reply_to_All [Ctrl-Shift-R]: replies to both "Reply-To" and "From" headers (Trevor and Roundtable). Sane.
- Reply_to_List [Ctrl-Shift-L]: replies to the "List-Post:" header (Roundtable). I didn't even know Thunderbird had this option until now!