Read/deleted messages not showing as such for same email, different location.
We have three addresses for our shop, and open Thunderbird on two computers and a mobile device. When emails are either read or deleted on one platform, they do not show as read or deleted on the other locations. (Had one mailbox showing 33 unread messages on one computer, but on the one they were opened, they were all showing as read).
Alla svar (4)
This is happening no matter how many times I restart Thunderbird.
POP or IMAP?
It is POP
Then it is working as expected. You need to use IMAP if you want synchronization with multiple devices. https://www.geek.com/chips/geek-101-pop-vs-imap-1536343/
If you decide to change protocols do not attempt to change the existing setup in Thunderbird. Follow this article to add the accounts back as IMAP. https://support.mozilla.org/en-US/kb/switch-pop-imap-account