what does the flag signify in Mail?

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