UBE (unsolicited bulk email) is a formal term for spam. The most common type of UBE is unsolicited commercial email (UCE): bulk email that is trying to get the recipient to buy some product or service.
Вангую, что SPF у тебя нет. Ещё список обычных подозреваемых: DKIM, reverse DNS, списки спамеров в сети.