Will email(s) be restored by the IMAP sync process after removal of the corresponding message file of an IMAP account from a Thunderbird profile?

Question

If one did the following on a client pc with an IMAP account:

• In Thunderbird: Have the synchronisation button (see "Synchronization & Storage" menu) of this account toggled on (so all messages are being fully downloaded, instead of "header information" only, and locally stored on the client pc to allow for offline use). Please note: this is all not about the "Local Folders" account in TB, neither are there mails in the "Local Folders" nor are any mails going to be put there. I am not referring to the "Local Folders" in any faintest way throughout this post.
• Close Thunderbird
• Remove a message file (mbox, eml e.g.) of the IMAP account from the Thunderbird profile folder (on the client pc)
• Keep all other files, including the respective *.msf file
• Start Thunderbird while allowing a connection with the IMAP server to establish
• Close Thunderbird

Would the message file removed be restored to the client by the synchronisation process? I am using Thunderbird version 102.8.0 under Windows 10.

I don´t just try because I can´t bring myself to risking severe trouble at the current state, and I couldn´t find nor figure out myself a reliable answer. I learned there are IMAP tags (something like message UID, deleted or read) and of course synchronisation, but I couldn´t find out where those tags are being kept and how the decision is obtained about which tag value (i.e. the one of the client above the server´s or conversely) takes precedence after a unilateral change of the tag´s value during an offline period or when files/tags are outright missing on one side (i.e. either client or server).

The cause of my question is my setting about giving Thunderbird a cleanup top down so as to eventually end up with a most reliable and easy to maintain installation, which I´m going to keep up from then on. My mails´ history is ancient, and they have been serviced by my two companions scant kwowledge and short time.

Answer 1

Thunderbird doesn't do "true" two-way sync. The IMAP server is authoritative for the folder contents; the messages stored in the Thunderbird profile are nothing more than a cache and can be safely deleted. State such as "read/unread" and "seen/unseen" is stored by the IMAP server as well.

(In general, even for local folders, the .msf files (Mork "mail summary file") are secondary and Thunderbird itself has a "Repair" button to outright delete them through a folder's "Properties" window, in case of the summary file becoming desynchronized with reality.)

If you want to be safe, you could remove the entire IMAP account from Thunderbird first, then re-add it as a new account after cleaning up the old files – because there is no pre-existing sync state for a newly added IMAP account, there is nothing else Thunderbird could do except load data from the server.

Answer 2

If I understand you correctly, you want to

• Move all desired messages to a Thunderbird Local Folder, not a folder on the IMAP server, on the PC, and
• Then delete any remaining messages in the account in Thunderbird,
• Then delete the account in Thunderbird.

That certainly will work, and you can test it to prove it to yourself -- just move the messages locally, disconnect from the internet, and see that they're still in a folder at the bottom, under Local Folders.

Because of the complexity of Thunderbird message storage, including indexing database, though, I suggest you do not directly remove files from the profile. Instead, remove all messages from that account folder by deleting them in Thunderbird, and wait for indexing to complete, before deleting the account. If you want to confirm that these message files have been deleted from the file system, you can then go into the profile in the file system and check that the relevant files are gone.

However, some messages contain links to the web, which might not be available offline. In the message below, in a local folder, note that the image (i.e., link) is replaced by a placeholder outline rectangle.

When you reconnect to the web and view that message, Thunderbird will give you the option to, "Show remote content in this message." As long as the external content is still available, it can be viewed, but there's no guarantee it will always be on the web. You might print to PDF any message you wish to preserve as it currently appears.