According to conditions 3 and 6:
All the letters written by Mary were marked.
According to conditions 1, 5 and 7:
Letters with only one page of stationery are all written in blue paper and are all collections.
According to condition 8 and the first condition we just got:
Mary wrote no more than one page.
According to condition 7 and the second and third conditions we just got:
All the letters written by Mary have been collected.
According to condition 4, we get:
I can't see Mary's letter because it is hidden!