You cannot '''block''' incoming emails because they are on the server and the download just downloads them. Those emails may have already been marked by the server as 'spam' and placed in the server Spam folder. It is possible to set this up via the webmail account, which may be more beneficial if you use POP as you would never see the emails downloaded to Thunderbird. But you might need to check the Spam folder gets emptied on occasions. If you use POP then you will not have access to the server Spam folder and so will not get them downloaded and the server will probably empty the server Spam folder periodically. If you use IMAP then you will get a download for all emails in all subscribed folders. Eg: Inbox and Spam. The best approach is to make sure the Thunderbird Junk controls are set and trained. https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages If the spurious emails are actually in the server Inbox, then you could set up a Message Filter to catch emails with Subject or From that contains certain text or email address and either mark as junk and put in the Junk/Spam folder or delete them, so they end up in Trash. Note Message Filters are usually best for organising emails. https://support.mozilla.org/en-US/kb/organize-your-messages-using-filters