Something that's forgotten isn't remembered or considered. Your forgotten mail might sit in the mailbox all weekend, if you simply don't think about it.
Your English teacher might reassure you that the bad grade you got on a paper last month is forgotten, and you could find that as you start reading a great book, your difficult day is forgotten. In both cases, something is overlooked, disregarded or erased from memory. Forgotten comes from forget, from the Old English forgietan, "fail to remember" or "neglect inadvertently."