If you have a Microsoft personal “Hotmail, Live or Outlook” account that is now “only setup as IMAP” in eM Client via the automatic account wizard as per Microsoft changes which started in August 2022 and finalized as of the 16th of Sept 2024.
See the following eM Client support forum link on that.
So if you have previously been using a Microsoft personal account with a POP setup in eM Client with a legacy username and password, that will no longer work, and you “now need to change to an IMAP” setup via the automatic account wizard in “Menu / Accounts” which uses OAuth 2.
So once you have your same MS account re-setup as IMAP, then drag / move all your existing POP messages and any custom POP message folders to your IMAP account. Then once you are certain they are all moved over, then remove your old POP account. You can see when that process is completed in Show Operations via clicking the drop-down on the right of Refresh top left.
Lastly before doing anything, make a manual backup of eM Client via “Menu / Backup” incase you need to restore for any reason. You can also see when the backup is complete in Show Operations.