Further information on Bill's email issue. Here are two past emails from him:
Sent Sun, 25 Apr 2021 20:21:16 GMT Subject line is BINGO! 64-bit! Fresh email, not a reply to anything, but has:
References: 201802171815.09906.gheskett@shentel.net In-Reply-To: 201802171815.09906.gheskett@shentel.net
in the headers.
Sent Mon, 26 Jul 2021 20:57:34 GMT Subject line is wrong timestamp - OT Fresh email, not a reply to anything, but has:
References: 826b2aec-5d94-a8e7-625b-2c883295dcff@caramail.com 202107241035.47688.mb_trinity_desktop@inet-design.com a84c0d35-9569-74b8-bfef-380ab701b98f@caramail.com In-Reply-To: a84c0d35-9569-74b8-bfef-380ab701b98f@caramail.com
in the headers.
Normally I would diagnose this as the sender hitting "Reply" to an existing email, but Bill assures us that he doesn't do that.
The signs do appear to show that when Bill sends a fresh email, something in the chain:
Bill's mail client --> sending mail server (his ISP?) --> any intermediate systems --> mailing list --> any intermediate systems --> my mail server --> my mail client
is messing up the References and In-Reply-To header. I'm confident it is not either my mail server or client, and I would be shocked if it were the mailing list or the ISP.