My wild guess is the connection is timing out. Perhaps your email account is a total mess being over quota. Perhaps your provider is the mess. I really do not know. However the server will be notifying Thunderbird that the information regarding the deleted emails is "out of date" so they are deleted. That is how IMAP works as the server is always considered the canonical source of information about each email. Why is Yahoo reporting this, your guess is really as good as mine. They are largely a failed internet company now owned by some sort of money management hedge fund. Internet know-how not required in that management office I would guess. You might look in the error console for errors that might offer clues as to what is happening. Not all errors actually bubble to the user interface if they can be recovered from (in this case by re-downloading the mail) so try the error console (Ctrl+Shift+J). In looking up things on Google about your post I encountered a reddit posting suggesting Yahoo have reduced storage for email account to 20GB See https://www.reddit.com/r/yahoo/comments/1lk3h1g/yahoo_mail_just_slashed_free_storage_to_20gb_from/ Are you perhaps responding to that change by trying to use IMAP to make some sort of backup? Previously I have heard that yahoo have some sort of IMAP limit to 10,000 email in an active IMAP synchronization set and that you have to use special archive servers for larger quantities of mail. I really don't know much about it. I wrote off Yahoo some 20 years ago when they showed their inability to manage a chat room and their groups product foundered in a mire of SPAM and mismanagement. However here is link to a discussion in that undocumented limit. https://www.reddit.com/r/Thunderbird/comments/v4ydu0/10000_email_limit_via_imap_in_yahoo_email/