Show Sulman's letter to Andover Potbello in New Sorpigal is the first quest in Might and Magic VI: The Mandate of Heaven. The party starts with this quest in their quest log.
The party attacked a goblin camp that ambushed travelers, and managed to drive the creatures away. Among the remains of the goblins' victims, the group found some letters from King Roland to his wife, and one from Xenofex to the traitor Sulman. Sulman had made sure that Roland's letters were never delivered, and that Roland was taken prisoner by the kreegans. Xenofex gave Sulman a letter and told him to talk to Andover Potbello for his reward, but Sulman was slain by the goblins on his way to New Sorpigal. The group decided to keep the letter and collect the reward in his stead.
This is the easiest quest in the game. The party starts outside the gates of the town. To complete the quest, they simply have to enter the A Lonely Knight inn (the first building on the right), talk to Andover Potbello, and show him the letter. He will give them 1000 gold. They will then receive the Bring Sulman's letter to Regent Wilbur Humphrey at Castle Ironfist quest.
This quest is technically optional, as the party can travel to Castle Ironfist and give the letter to Wilbur Humphrey immediately, but since the quest gives a decent amount of gold for no effort, there is no reason not to complete it.