Today, Thunderbird started issuing new mail alert sounds every minute. I turned off "Play a sound" and it stopped. I have one email account set to look for mail every thirty minutes. The rest of the accounts are all set for manual only. There are no notifications associated with the sounds. There isn't any reason for the sounds that I can find. I'm running Windows 10. Restarting Thunderbird made no difference and it is up to date.