Use ～せいで to attribute an undesirable result to a cause, with an implicit tone of blame or fault-finding. In business writing it appears in incident reports, apologies (blaming one's own oversight) and explanations of missed targets — though pointing at an external party can sound accusatory, so 「～の影響で」「～により」 are often preferred in outward-facing email. Contrast with ～おかげで (favorable outcome) and the noncommittal ～せいか ("perhaps because").