Feeding horses a few slices of raisin bread as a treat is not harmful for them, in fact there seems to be some evidence that sultanas are good for horses, as they can be good for humans, with their anti oxidant properties.
The key with any treats, is moderation, so that the basis of a horse's diet is made up of a balanced and nutritious diet. Excess treats are not balanced. Mind you horses each so much, it would be positively willful and perhaps neglectful to attempt to feed a horse an excess of treats.
Some horses owners are concerned about feeding their horses raisins because they have been shown to be toxic to dogs and some other domestic animals, however this does not apply to horses. Raisins are perfectly acceptable treats for a horse so long as they are fed to them in quantities of less than 1-2 lbs.
Naturally Healthy Horse Treats By Katie Shoultz discusses fruits that are safe for horses and also endorses that raisins are safe for horses.
This answer discusses that feeding bread to horses in small amounts (as treats) is not harmful. So I will address the sultana/raisin part of the bread.