It is in fact part of a unique series of portraits by Van Gogh of an entire family, 'The Roulin Family', who have since become somewhat famous themselves as a result.
Though Van Gogh of course loved to paint, he was also poor. This made it hard for him to find and convince models to sit for him, which is what made images such as this one a rare occurrence during his career. Joseph Roulin became a good and loyal friend to Van Gogh during the artist's stay in Arles, France, in 1888-89. This relationship meant that Van Gogh's portrayal of his friend was extremely important to him, as he would've wanted to do him justice.
The portrait itself shows the artist's typical talent for bringing things to life, whether it was a beautiful natural landscape or an honest reflection of harsh reality. He told colourful stories through painting that movies wouldn't come close to achieving for many years to come. In this case, we see reflected back at us the vision of a weathered yet proud individual, most likely a man who worked hard for his family and was immensely happy to do so.
Van Gogh captures the aging effects on Roulin's face, showing a man likely in his 50's with this reflected by the lines on his skin. He also showcases an impressive beard that many other men would be envious of; one that was seemingly once a light brown but is now showing gradual signs of grey.
Joseph Roulin seems a man who took pride both in his appearance and in commitment to his work. He holds himself fully upright with a confident, assured look in his eyes while his striking blue uniform appears to be in pristine condition, kept clean and fully buttoned up. We get a sense of the respect Van Gogh must have had for Roulin, as well as the respect likely shown by his co-workers. This looks and feels like a man who could be trusted and relied upon to give his best in any given situation.
Bearing all of this in mind, Portrait of the Postman Joseph Roulin may well have been Van Gogh's most personal work, or at least near the top. It tells the story not only of a strong, confident individual but of a true friendship lurking under the surface of the canvas.