re :Until now I always used POP3 for all my accounts, deleting some of the emails from the server after moving them to my local folders.

Please note: All Pop accounts do not synchronise with any server folders, they merely get access to download from server Inbox to pop account Inbox. So providing you downloaded full copies and not only headers then they were already stored on your computer in the Pop mail account. You did not need to move them to 'Local Folders' account unless you were going to Remove the Pop account and wanted to keep seeing emails.

You cannot simply swap the server settings of a pop account to use imap server settings as they functions very differently and it may cause servious issues.

So - I presume you moved all the POP account emails into the 'Local Folders' mail account. Then you completely deleted/removed the Pop account from Thunderbird. Finally, you created a new mail account for the existing email address and this time set it up as IMAP.

IMAP is not like POP. Good info here: https://support.mozilla.org/en-US/kb/imap-synchronization

Emails are stored on the server. IMAP account folder(s) sycnhronise with server folder(s) of same name to display what it sees on server.

You can create new folders in an imap account and set up to subscribe to see them and they will get auto put onto the server. You can also logon to webmail account and create new folders. Then in IMAP account choose to subscribe to see those new folders.

When you move an email from one imap folder to another imap folder on same account, this synchronises with server to update server. If you delete an email, email goes to imap Trash and server gets updated. If you move an email from an imap account folder to 'Local Folders' account, then you are moving that email off the server - in effect you are deleting the email off server. Although, you may find servers like Gmail or who use same 'label' structure will only delete the 'label' and not the email because email was not put into the server Trash folder. Likewise, if you move an email from Local Folders to an imap folder, it will perform a synchronise with server to upload back onto server.

Please note: if you should ever want to move emails off or upload back on to a server, I would advise you use 'Copy to' and not 'Move to'. If something happens in transit you may lose the email, so always use Copy, then if something goes wrong you have not lost the original email.

There may be a limit in how much you can upload back onto the server. This may vary but as an example gmail says : IMAP upload bandwith limits: https://support.google.com/a/answer/1071518?hl=en

re : It would be nice to copy back the emails deleted from the server that are now in my TB local folders only. If you are putting emails back onto the server this will use up some of your server 'Quota'.

The only advantage of putting emails on server is that you can access all of them from any imap connection or webmail.

If 'Local Folders' account has a load of 'older' emails which you are not really likely to need but you want to keep just in case, then I would not bother putting them back on server as they are just using up quota for no real purpose. Consider 'Local Folders' like an archive. Please note any emails in the 'Local Folder' mail account are independent copies stored on your computer; there is no synchronising with any server. If you have some very important emails then 'Local Folders' is an ideal place to put a copy.

Please remember, in an IMAP account you can choose to download full copies or headers only on a per folder basis. You cannot believe that any IMAP account is a separate independent copy (POP accounts were separate independent copy). This is because all imap foldres synchronise with server, so if for some reason you lost emails off server, then any imap account that synchronises with server will also appear to lose emails.

Which means, I would strongly advise that periodically, you fully synchronise all folders - then exit Thunderbird and get a copy of the C: Users/User name/Appdata/Roaming/Thunderbird folder and keep a copy in an external drive.

Alternatively there is an addon 'Import ExportTools NG' which offers ability to backup profile - make sure to fully synchronise all folders and go into 'Offline' mode to stop activity whilst backup is created.

• https://addons.thunderbird.net/en-us/thunderbird/addon/importexporttools-ng/

In the new version 102 which has just been released, I believe it will contain Import/Export Tools to assist with making a backup of profile.

• https://support.mozilla.org/en-US/kb/new-thunderbird-102

Please note: If you have an Anti-Virus product which is allowed to scan Thunderbird profile folders or scan any file opened in your profile or scan any incoming/outgoing emails:

If AV finds a problem, make a note of the problem but do not allow any auto fixing by an Anti-Virus. Emails are stored one after the other in mbox text files, so any folder you see in folder pane is actually a file. If that file gets fixed, you could lose contents meaning all your emails because AV do not understand that the file is not a single email - it may contain hundreds of emails. Remember imap folders synchronise with server.

I suggest you set up AV to ask what to do, so you have the opportunity to manually fix and block an auto fix.

Bad emails can be manually deleted - then folder compacted to remove all old traces of email. Providing you follow simple rules, you shouldn't have any real problems. eg: Do not automatically allow remote content to be displayed - by default Thunderbird does not display it. Do not open attachments because you are curious. Nefarious people may send eg: 'invoices' because they are banking on you being curious enough to open it. Do not automatically switch on 'Display Attachments Inline'. Use that option when you really know who sent the email and you want to see the image the person sent.