Things are still not right, I will try to start from the start so that the problem will become “clearer” (if it can)
I had no problem with my mail on my old computer, it ran Windows 10 and Office 2010, Outlook was set with “pop”. In Outlook I had kept locally all my received mails since 2009 and all my sent mails since 2015. A good part of these mails were organized in local (on my PC and not synchronised) personal folders. I had noticed when I used the webmail to access my mail (on a tablet during vacation for instance) that the messages I sent with Outlook were not stored on the server.
My computer being 11 years old, I had to get a new one in order to switch to Windows 11. I seized the opportunity of this switch to a new computer to upgrade to Office 2021 (the last version which one could buy and not rent.)
I exported the emails I had in Outlook 2010 and imported them in Outlook 2021. I then received the message that my mailbox was more than 80% full! I went on webmail and discovered that in spite of my “pop” setting Outlook had loaded all my folders and all the mails in my “sent” folder into the server mailbox!
In order to make space still using the webmail I erased the mails in the “sent” folder, only to find out that
1. The space had not been released,
2. That all the “sent” mails in my local PC had been erased!
I then in Outlook unsubscribed those folders I was keeping locally, they disappeared from my PC!
I went back to the webmail in order to make space and got rid of 11 years of emails on my inbox and 5 years of emails on my “sent” and removed what had been the local folders. The space was not released. I then removed additional mails using Outlook, the space was released! I checked in Outlook: The local folders were gone but they were still defined as “unsubscribed” so I imported them again. I got again the message that I was using more than 95% of the space in my mailbox! I checked: Instead of having been imported like the first time, the folders had been this time imported into “Archive” and thus were again “subscribed”.
I got them out of “Archive”, erased them on the server, it seems to work but I don’t know what to do with the “sent” folder and wonder whether my “local” folders will still be there tomorrow.