We're running GNU Mailman 3.3.8 (Tom Sawyer) on Debian Linux.
See below for what mailman.log had repeatedly. I removed the message from the hyperkitty spool directory again just before lunch today and it hasn't come back again. That may have done the trick. This is the first time I encounter this kind of issue. I looked at the .pck via mailman qfile and it definitely looked like some kind of junk mail from Microsoft so we don't need to recover it.
"Jan 04 10:54:10 2024 (31780) archiving failed, re-queuing (mailing-list departmental_directory.caltech.edu, message <[1db292f04b54496ab40544b0989a2122-JFBVALKQOJXWILKNK4YVA7CPNZSVMZLUPRCW2YLJNRKGK43UPRCXQ32TNV2HA===@microsoft.com]>) Jan 04 11:25:11 2024 (31780) archiving failed, re-queuing (mailing-list departmental_directory.caltech.edu, message <[569decf8204e4b869f4501ed8fbe379d-JFBVALKQOJXWILKCJQZFA7CPNZSVMZLUPRCW2YLJNRKGK43UPRCXQ32TNV2HA===@microsoft.com]>) Jan 04 11:26:03 2024 (31780) archiving failed, re-queuing (mailing-list departmental_directory.caltech.edu, message <[569decf8204e4b869f4501ed8fbe379d-JFBVALKQOJXWILKCJQZFA7CPNZSVMZLUPRCW2YLJNRKGK43UPRCXQ32TNV2HA===@microsoft.com]>) Jan 04 11:27:52 2024 (31780) archiving failed, re-queuing (mailing-list departmental_directory.caltech.edu, message <[569decf8204e4b869f4501ed8fbe379d-JFBVALKQOJXWILKCJQZFA7CPNZSVMZLUPRCW2YLJNRKGK43UPRCXQ32TNV2HA===@microsoft.com]>) Jan 04 11:29:01 2024 (31780) archiving failed, re-queuing (mailing-list departmental_directory.caltech.edu, message <[569decf8204e4b869f4501ed8fbe379d-JFBVALKQOJXWILKCJQZFA7CPNZSVMZLUPRCW2YLJNRKGK43UPRCXQ32TNV2HA===@microsoft.com]>) Jan 04 11:31:02 2024 (31780) archiving failed, re-queuing (mailing-list departmental_directory.caltech.edu, message <[569decf8204e4b869f4501ed8fbe379d-JFBVALKQOJXWILKCJQZFA7CPNZSVMZLUPRCW2YLJNRKGK43UPRCXQ32TNV2HA===@microsoft.com]>) Jan 04 11:33:08 2024 (31780) archiving failed, re-queuing (mailing-list departmental_directory.caltech.edu, message <[569decf8204e4b869f4501ed8fbe379d-JFBVALKQOJXWILKCJQZFA7CPNZSVMZLUPRCW2YLJNRKGK43UPRCXQ32TNV2HA===@microsoft.com]>)"