Settings → Mailboxes → Add → Microsoft. OAuth with the sending account; tenant admins may need to grant consent once for the organization. Same domain guidance as Gmail: use a secondary sending domain.
Steps
Settings → Mailboxes → Add Mailbox → Microsoft 365. Sign in and accept permissions (Mail.Send, Mail.Read, offline access). If your tenant requires admin consent, the flow generates an admin-consent link to forward.
Tenant settings that block connection
Conditional-access policies and disabled third-party app consent are the two common blockers. The error screen identifies which one fired; both are resolved in the Entra admin center.
Shared mailboxes
Shared mailboxes are supported for sending with appropriate delegation; reply detection works but health signals are noisier. Prefer individual licensed mailboxes for outbound.
After connecting
Warm-up starts automatically. M365 mailboxes on brand-new tenants warm slower — expect the conservative end of the ramp schedule.
