Any ideas? I guess roll back to the last version, but before I spend time on that, I wanted to see if this resembles any known issue.

For years I've used a Linux server (Ubuntu 14.04.5 LTS) as a personal email server, just for me and one other person. Postfix 2.11.0, Dovecot 2.2.9. This weekend I got a notification about a huge about of network traffic on the server: "averaging 17.04 Mb/s for the last 2 hours". That's about a gigabyte a minute, and there was no good reason for that to be happening. After various debugging (tcpdump and nethogs on the server; network monitor on my client machine, then leading me to Thunderbird activity monitor), I found that Thunderbird was repeatedly saying it was downloading x of 98 messages. Except all the messages appear to be already downloaded. It appears to be working normally, other than this activity I can see in logs and activity; the messages match what is in other clients, new messages come in normally. Client is Thunderbird 52.5.2 on Windows 7. The same Thunderbird install also is connected to another mail server, and it is behaving normally for that account. Safe-mode Thunderbird still has the same traffic, so it is not caused by a plugin/extension. Client and server both have adequate disk space. Any ideas? I guess roll back to the last version, but before I spend time on that, I wanted to see if this resembles any known issue.