I suggest you keep doing what you're doing: copying the folders you need. You then don't need the Allmail folder.

Hi David,

thanks for replying. As not all the emails are in folders and I want to have access to allmail as an archive, your suggestion would not be an ideal solution for my challenge.

br, Fred.

I have experimented with Google takeout in order to see if this could be a solution to my challenge. However, google takeout creates an inbox which includes all the replies sent - and contains almost twice the amount of emails as my original inbox.

On another note, I will try to copy all the folders (using Thunderbird) and thereafter run the extension "remove duplicate messages" (https://addons.thunderbird.net/en-US/thunderbird/addon/removedupes/). This may very well be the solution.

Who is your new provider? most of them offer some form of IMAP transfer or import. Google certainly offer a takeout service that provides the contents of the account as a series of mbox files. Perhaps you new provider offer a way to import those to the server if the do not offer something more robust.

While there is a cottage industry of migrating mail from account to account with a mail client, it is slow, buggy and prone to error. Data loss is not uncommon.