So, how can you tell Mozilla Thunderbird not to mark messages read if they only appear in the preview pane? Sadly, you cannot. You can tell it not to mark them read immediately — and not for a long time —, however. Usually this is good enough.
Prevent Mozilla Thunderbird from Marking Mail Read in the Preview Pane
To make Mozilla Thunderbird mark mail in the preview pane read only after a (considerable, if you desire) delay:
- Select Tools | Options... from the menu in Mozilla Thunderbird.
- Go to the Advanced category.
- Make sure the General tab is selected.
- Now make sure Wait ___ seconds before marking a message as read is checked.
- Enter the desired delay (up to 999 seconds or 16 and a half minutes).
- Close the preferences window.