Why can horses and donkeys mate?

That's not always the case with donkeys and horses, which are of two different species. Usually, it's the male donkey (jack) mating with a female horse (mare) to produce a mule, which can come in either sex. read more

Mating can take place between any two related or unrelated species.In other species fertilisation doesn't take place , but in a cross ofmale horse with a female donkey fertilisation also takes place inspite of different genomes, but the offspring's produced are sterile.This is an exceptional case of its own type. read more

A female horse and a male donkey have a mule. But hinnies and mules can't have babies of their own. They are sterile because they can't make sperm or eggs. They have trouble making sperm or eggs because their chromosomes don't match up well. And, to a lesser extent, because of their chromosome number. A mule gets 32 horse chromosomes from mom and 31 donkey chromosomes from dad for a total of 63 chromosomes. read more

The specific species definition referenced in the OP states that individuals of separate species cannot mate and produce fertile offspring. Horses and donkeys mate to produce mules, which are sterile (most of the time - there are occasional exceptions), so they do not violate that particular species definition. read more