why do some messages appear in bold in sent file

This is actually pretty normal behavior in Thunderbird! Bold messages in your Sent folder simply means those messages have not been opened/read yet after sending. In Thunderbird, bold = unread in any folder — including Sent. Once you click on that message to open it, it will go back to normal (non-bold) text. If you want to mark them all as read at once: Click on your Sent folder Right click on it Select "Mark Folder as Read" All bold messages will become normal instantly ✅ To stop this from happening automatically in future: Go to Edit → Preferences (or Tools → Settings) Click on "General" Look for "Mark messages as read" option You can set it to mark messages as read automatically after a few seconds That's really all it is — nothing wrong with your Thunderbird, just how it displays unread messages! 😊 Hope that helps!