Several updates ago, Thunderbird does not seem to be able to correctly mark each message as "read". This issue did not seem to occur with any frequency some months ago (>1%), but since upgrading to the latest versions this error now occurs with a decent percentage of emails (~10%). Automatic marking of emails as "read" after being viewed for some time does not work on these emails; I can be sitting waiting for several minutes, and these emails are never marked as "read" despite most other emails being correctly marked as they are viewed, typically within seconds. Manually marking these emails as "read" does not always work either; these messages are quickly reverted back to "unread" status, and it can take several attempts (and a couple minutes) to mark an individual message as "read". It appears that the latest updates have changed the way these read/unread flags are handled and communicated with the mail server, in a way that is not consistent from one message to the next.