In a large bowl, dissolve yeast in warm milk. Stir in 1 cup flour and 1/2 cup sugar. Cover bowl with plastic wrap, and set aside for 1 hour. Meanwhile, steep mahlepi in 1/2 cup simmering water. Let stand for 5 minutes. Strain, and discard mahlepi. Let cool.
Add mahlepi infusion, butter, and 5 eggs to yeast mixture; combine thoroughly. Add orange and lemon zests. Sift in remaining 8 cups flour, sugar, and salt; stir with a wooden spoon until dough forms. Knead dough in bowl until smooth, about 10 minutes. Form dough into a ball, and transfer to a lightly buttered bowl. Cover, and let rise for 2 hours.
Punch dough down, and transfer to a lightly floured surface. Divide dough into 6 equal pieces. Roll each piece of dough into a rope, about 15 inches long. Tightly braid 3 ropes together, beginning in the middle. Repeat with remaining 3 ropes. Tuck a red-dyed egg into the first and last plaits, using a small piece of coiled dough to hold each egg in place if necessary. Place each loaf on a buttered baking sheet, cover loosely with plastic wrap, and set aside to rise for 1 hour.
Preheat oven to 350 degrees F.
Beat remaining egg with 1 tablespoon water. Brush bread with egg wash, and sprinkle with sesame seeds. Transfer to oven, and bake until golden, 50 to 60 minutes.
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