The padlock indicates a secure connection setup for that account.

If that account has it's Connection Security set to none there will be no lock. Find out if your provider supports something different and what the setting is supposed to be. Then make those changes in Thunderbird.

Thank you so much! I am a novice at this and have a struggle finding things. You did a great job of straightening it out and explaining what to do. You made my day!

Same idea as when you visit a secure website like this forum that has https at the beginning. S is for secure and there is he padlock.