[https://support.mozilla.org/en-US/kb/imap-synchronization Mozilla's documentation on IMAP Synchronization] Mozilla's documentation on IMAP Synchronization indicates "Non-synchronized messages are still accessible within Thunderbird. The only difference is that rather than downloading the entire message, only the message "headers" are downloaded. (Message headers contain information like the sender, recipient(s), subject, etc - everything except the body of the message.) The message body is not downloaded until you click on the message in the message list" Under what settings (e.g. Sync messages checked or unchecked) does this behavior apply? Thanks.