Hackers Favor US Eastern Business Hours in M365 Phishing Campaign

KnowBe4 tracked 29,785 phishing emails abusing Microsoft 365 Direct Send to spoof internal senders while timing sends to US Eastern business hours.

KnowBe4 Threat Lab observed 29,785 confirmed phishing emails between July and August 2026 abusing Microsoft 365's Direct Send feature, which lets devices and legacy apps send mail without a dedicated account. The emails appeared to come from trusted internal addresses such as HR or accounting, bypassing email gateways by connecting directly to Exchange Online MX endpoints, with activity peaking on Monday-Tuesday during US Eastern business hours and near-zero weekend volume. About 35% of the messages carried malicious attachments like fake invoices, voicemail alerts, and OneDrive shares, and 4,023 used reply-to addresses on different domains to capture employee responses. Researchers recommended strict DMARC enforcement, connector restrictions, DKIM signing, and checking for the 'X-MS-Exchange-Organization-AuthAs: Anonymous' Exchange header.