Sorry if this post is a little off-topic but it may be of interest here.
I had some fun working out where this cover had been.

It was sent in 1851 from Ottoman-controlled Crete to Marseille via Trieste, Milan and Switzerland:


I believe the following is the story of its journey:
1. Chania, Crete, Ottoman Empire - 8th January
- Letter heading inside is actually dated 28th December (i.e. Old Style) but I guess this was a mistake for the 27th.
2. The island of Syros, Greece - 8th January
- Black "SYROS" (in Greek) CDS at right on the front, dated 27th December 1850 (Old Style)
- The black "DP" (in Greek) mark at the top right on the front indicates it was paid to its destination.
- Syros was the major shipping port in the area before the rise of Pireaus so its mark often turns up on early covers.
3. Trieste, Austria - 12th January
- Black "TRIEST" CDS at top right on the back
4. Milan, Lombardy-Venetia - 14th January
- Red "MILANO" mark in the top middle on the back
- I wonder if this is where the black "AT" (Autriche Transit) mark at the top on the front was applied before it was sent onwards to Switzerland.
5. Chiasso, Switzerland
- Red "VIA DE CHIASSO" mark at the top on the front.
- This was applied to letters from Lombardy-Venetia bound for Basel.
6. Basel, Switzerland
7. Saint-Louis, France - 16th January
- Red "AUTRICHE / BEAU FR 1 DE BALE" CDS at top left on the front
8. Marseille, France - 19th January
- Black "MARSEILLE" CDS bottom middle on the back