I decided on enabling ufw (uncomplicated firewall) on my laptop (you can’t be secure enough). Too bad it isn’t enabled by default on Ubuntu. So I had to do this:
sudo ufw enable sudo ufw default deny sudo ufw deny "Dovecot POP3" sudo ufw deny "Dovecot IMAP" sudo ufw deny "Dovecot Secure IMAP" sudo ufw deny "Dovecot Secure POP3" sudo ufw status verbose