Many users have had issues with this failure of emails to be sent. All seem to apply to all outgoing emails. I have occasional problems with only some emails. I have checked Thunderbird settings on the outgoing server (SMPT). They show correct user name and show authentication method: Normal password. I have noticed that the isp password in windows 10, under settings - accounts-email-email& accounts shows a hidden password with too many dots for my current password, but I don't use any windows mail. Where is the password stored that TB uses for authentication? Is there a way to confirm an exact reason for these emails to be rejected? My current problem is with a website in Europe.