Guild Wars 2

I have no idea what caused it, but since yesterday on one of my characters I keep repeatedly getting the ‘History Lesson’ mail.
I have not followed story lines linearly, thus have not completed main story line, but have already marked the ‘history lesson’ as completed.
Yesterday I have deleted 3 instances of the same message, today I have 10 more in the last 3 hours (probably generated on map or character change)

You could try a -repair of the client. Instructions found in the Knowledge Base or the Wiki.

Good luck.