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Adding a new domain to Microsoft 365 without email downtime

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Adding a new domain to Microsoft 365 without email downtime

How to add an email domain to Microsoft 365 without downtime: verification, accepted-domain type, SPF, DKIM, DMARC before MX, and the mail-flow traps.

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-add-domain-without-downtime

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Step 1: verify ownership, and nothing else

Microsoft 365 admin center → Settings → Domains → Add domain. Add the TXT verification record at the registrar and verify. Stop the wizard here — choose to configure DNS yourself, not to have Microsoft add records. Veri…

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-add-domain-without-downtime

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Step 2: decide the accepted-domain type

Exchange admin center → Mail flow → Accepted domains. The verified domain appears as Authoritative by default: Exchange Online believes it holds every mailbox for the domain and rejects mail to any address it does not k…

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-add-domain-without-downtime

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Step 3: authentication records, before any mail is sent

Set these up while the MX still points at the old system. None of them affects inbound mail.

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-add-domain-without-downtime

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Step 4: stamp addresses on users

Add the new domain as a proxy address on the users who need it. Whether it becomes the primary SMTP address depends on the business; a rebrand changes the primary, an acquisition often keeps the old one primary for a wh…

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-add-domain-without-downtime

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Step 5: Autodiscover, and a test

Create the autodiscover CNAME to autodiscover.outlook.com. If the old platform still serves Autodiscover for the domain and users' clients are still there, hold this until the mailboxes have moved — Outlook profiles tha…

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-add-domain-without-downtime

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Step 6: the MX, at a quiet hour

Lower the TTL on the existing MX to something short a day ahead. Then change the MX to the value the admin center shows for the domain (domain-com.mail.protection.outlook.com), keeping the old MX briefly as a lower-prio…

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-add-domain-without-downtime

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Downtime, honestly

None, if the MX is last and the aliases are stamped before it moves. The realistic risk is not lost mail but delayed mail during propagation, and mail from the new domain being treated with suspicion by receivers for a…

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-add-domain-without-downtime

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