My computer keeps giving an alert sound similar to a new email that arrived in Thunderbird but when I look there isn't one. I don't know which program is alerting me or why. System - Ubuntu 20.04.3 LTS. Firefox web browser, Thunderbird mail client, LibreOffice, and Gimp are my most frequently used programs. My question is how do I find out the cause and how do I stop it?
Solution - I have found the cause. Thunderbird Mail was giving an alert every time it checked for emails, whether it found one or not.
