Maybe part of the problem is because your 'Multifloor Gdansk' account has more than one folder designated as a sent folder, so perhaps there is some confusion going on. I suggest you start by getting this sorted first. In 'Multifloor Gdansk' account you seem to have two 'Sent' folders. # Elementy wyslane (Items Sent) # Wyslane (Sent) You should not have two sent folders. One is the default folder used by the server and that is the folder you need to use. But which one is the correct one used by the server ? 'Elementy wyslane' (Items Sent) OR Wyslane (Sent) ? Log on to your webmail account via a browser and send a test email to another email address, which folder does the server auto put the copy of that sent email. Is copy of sent email in the 'Sent' or the 'Items Sent' folder ? Whatever folder received that copy of webmail sent email is the folder you need to use in the imap account in Thunderbird. In Thunderbird: 'Account Settings' > 'Copies & Folders' for the 'Multifloor Gdansk' account Select 'Place a copy in': Select 'Other' and choose the correct folder - the one the server uses by default. Then you need to remove the other one as it causes confusion. If there are sent emails in the wrong sent folder then move them to the correct sent folder. Then compact that wrong sent folder. Finally, remove the wrong sent folder by unsubscribing to see it. In Folder Pane : * Right click on 'Multifloor Gdansk' account folder and select 'Subscribe' (Subskrybuj) * Select the sent folder which is not used by server * Click on 'Unsubscribe' (Zrezygnuj z subskrypcji) * click on 'OK' This should then mean you only have the one sent folder. See if this improves the saving of sent copy.