Run email for many domains on one Microsoft 365 license
Microsoft 365 bills per license — not per domain and not per mailbox. That single fact is the whole idea behind inboxroot.
Two features of a tenant are free: accepted domains (one tenant can hold hundreds) and shared mailboxes (free under 50 GB, each with its own inbox and send identity). Grant your one licensed account access to a shared mailbox and it mounts natively in Outlook.
So each new domain becomes a free shared mailbox in your existing tenant: native Outlook send and receive, a separate inbox, and zero extra Microsoft cost. inboxroot automates the whole flow — adding the domain, generating DNS, verifying ownership, creating the mailbox, and granting access.
Postmark rides alongside for transactional mail (receipts, notifications) on the same domains, because it signs with DKIM and a Return-Path record and needs no MX — so it never conflicts with Microsoft receiving your human mail.