<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">Hi there,<br><div><br><blockquote type="cite"><div>On 16 Dec 2022, at 17:13, William Herrin <bill@herrin.us> wrote:</div><br class="Apple-interchange-newline"><div><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;">Most email clients assume that a change to the subject line (other</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;">than adding "Re:" to the front) indicates that the sender wants to</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;">discuss a new topic related to but meaningly different from the last.</span></div></blockquote></div><br><div>Although I generally agree that changing the Subject line without reason is an annoyance, I didn’t notice any issue with it until I came across this thread, which wasn’t broken in my mail client (Apple’s Mail.app).</div><div><br></div><div>This led me to a few tests, and FWIW even Mutt seems happy with the Subject changing and still threads the emails appropriately.</div><div><br></div><div>In my experience, threading is done by clients looking for the In-Reply-To: header, not subject. Subject is a heuristic fallback, in case In-Reply-To is absent.</div><div><br></div><div>Some email clients (although I don’t remember which ones) remove In-Reply-To: when the Subject: is changed (that might go as far back as my Gnus Oort days).</div><div><br></div><div>BR, ic.</div><div><br></div></body></html>