IMAP has always required manual intervention (timed fetching) for the vast majority of new mail. When IMAP was first released, it was the only way to get new mail and even now it renews the login that is used to push out mail using IDLE. The IMAP [https://datatracker.ietf.org/doc/html/rfc2177 Idle command] allows the server to push new mail. In practice this is sometimes only new mail in the inbox, so server side filtering can mess things up, depending on the server implementation. Thunderbird does not routinely poll the junk/spam or deleted/trash folders and that is a default set by developers as interruptions notifying of getting spam is hardly a good thing. You can modify that and include the folder by right clicking it, selecting properties and checking the box to include the folder when getting mail. Note that there have been issues with mail more than two levels below the root folder, I did not follow the bugs and I am not sure if they are fixed. But from what you have offered I does not sound like your issue. Note that when there is an antivirus product scanning mail it sits between the mail client (Thunderbird) and the server pretending to be either the server to Thunderbird or a client to the server. This greatly increases the chance of failures, so disable that if it exists to see if it may be a factor. I can see an antivirus product simply dropping the ball completely with server initiated data downloads.