This has just started since I adjusted some of the category colours.
The flag indicates that you want to follow-up an e-mail at a later point. This causes it to be displayed in the agenda panel at the right-side.
You can remove the flag by doing one of the following:
Clicking on it (while viewing the e-mail in the e-mail list)
Highlighting the e-mail and pressing Insert
Highlighting the e-mail and going to Actions -> Follow Up in the menu