It might be the case that when somebody sees this painting cannot really tell the letter the woman in yellow is given is a love letter. However, I have found a theory which briefly offers a very interesting and wit hint to let unexperts in art like me realise that the letter in the drawing by Vermeer is in fact a love letter. I must say that I certainly think the whole idea this theory maintains is absolutely surprising since I would never have thought a painter would pay atention to this type of details that surely enough will not be noticed by many viewers who lack the artistic knowledge this requires. Here follows the possible explanation:
Although the lady has not yet opened the letter it is apparent from the picture that it is from a lover. The two pictures in the background indicate this. A painting within a painting often indicates the artist’s intention in the picture. Here the lower painting is a seascape. In the seventeenth-century language of imagery the sea stood for love, and a ship for a lover. The emblem written by Jan Krul ‘Far from home, never far from my heart’ expresses this well. The upper picture shows a man walking along a sandy path: as in the painting of the ship, there is the suggestion of a person on a journey.
Emblem by Jan Harmensz Krul, Far from home, never far from my heart, 1640