Have you tried just adding the account as an IMAP O365 account, without touching the Gsuite account? As the mail is stored on the IMAP server, the folders should sync correctly. Otherwise, your method of changing the server settings might work if you also delete the existing passwords in Edit/Preferences/Security/Passwords/Saved Passwords, restart TB, and enter the password for the new server when prompted.

Thank you @sfhowes for your response.

1. Adding a different O365 imap account works and mails are synced correctly. 2. Similarly, changing the server settings on existing Gsuite account to point to O365 and changing the password works pretty well but the existing Gsuite is reset and mails are refetched.

However, I would like to avoid adding new O365 account as in option 1 and re-use the existing Gsuite folder with O365 account without refetching emails again from O365 as in option two.

Any further ideas?

Thank you in advance

I don't think there's any way around re-fetching the mail in the new account, as that is how IMAP works. But you can limit the downloads to headers only by unchecking 'Keep messages in all folders...' in the Synchronization & Storage section of Account Settings. If you go offline from File/Offline/work Offline before you add the account, the box can be unchecked in the Advanced config. of the account setup, so that when you go back online and connect to the server, only headers are downloaded. Message bodies are only downloaded when they are opened, stored in the cache, and no mbox file, where TB normally keeps messages for offline access, is written.

Thank you for your response. Will try and see how it go. Much appreciated!

Hi, guys! kmibey.10, does the following reflect a correct reading of your problem?

1. You have all of your old e-mail on the Office 365 server now (moved or copied out of your old GSuite server).
2. You also have Thunderbird, the mail client, and you want an account folder there, set up to sync via IMAP to Office 365, from now onward. Just one :-) .

What exactly *is* the problem with re-downloading the mail from Office 365 for a good sync? (I'm not saying there isn't one. I assume there is, because you say so :-) , but I don't know why it is problematic. That answer might inform my attempts at help here.)

NOTE: When I say "folder" or "account folder" below, I mean "account folder, including all subfolders and other contents".

My (maybe-wrong) presumption is that with IMAP syncing, the states of the stuff in Office 365 should have matched the prior states of things in Gsuite at the moment after migration. So (ideally), the stuff in the Thunderbird account folder should have already matched that state as well. And you'd have had to do nothing but change the server settings and e-mail on your merry way, so to speak.

BUT, Thunderbird is re-fetching e-mail. So, is the problem that you're getting duplicates of letters in your Thunderbird folder? Even if that's the case, I wouldn't think there would be any problem after the initial download, beyond clearing out the duplicates (tedious if intermixed with the originals, no matter how you sort). BUT, if you were to just set up a new Thunderbird account folder synced to Office 365, why wouldn't that simply serve you as a perfectly usable replacement for everything in Thunderbird's old GSuite account folder? Genuine question and curiosity there. They should just be an exact match. The only answer I can come up with on my own is that maybe you have dragged other things into that GSuite account folder, and it's extra work to find and move those again??? But I happily await your actual answer, too :-) .

Could you allow the one-time download into a new folder, and delete the results (or all but those that came in after migration between servers), then drag everything wholesale from the old folder into the new folder, then delete the old folder ... without problems for you?

I don't know what the problems for you are, but I can imagine some. For instance, limited bandwidth or having to pay for each byte downloaded (metered bandwidth), would make it ... stink ... to have to download a ton of old mail. But if it's just a matter of, say, the time, letting it happen and doing something else meanwhile, then perhaps it's only a one-time pain you won't have to deal with later.

(Or if you're like me, maybe the actual situation is already handled, but it's still good to know for the future and to expand your knowledge. I'm fascinated to follow this particular thread, and help if I can.)