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Exchange Online mail flow and connectors

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Exchange Online mail flow and connectors

How mail moves into and out of Exchange Online — connectors, transport rules, and hybrid routing.

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-mail-flow-and-connectors

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01

When do I need a connector in Exchange Online?

When mail must flow between Exchange Online and something that is not the internet at large: an on-premises Exchange server (hybrid), a third-party email security gateway, a smart host, or devices and applications that…

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-mail-flow-and-connectors

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How do I let a printer or application send email through Microsoft 365?

Three options: SMTP AUTH client submission with a licensed mailbox (simplest, needs SMTP AUTH enabled for that mailbox and modern auth is required); direct send to your own tenant only (no auth, internal recipients only…

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-mail-flow-and-connectors

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03

Why is my mail rejected with 'Relay Access Denied' or 5.7.x errors?

The sending system is not authenticated (SMTP AUTH off or disabled for the mailbox), it is sending from an IP or certificate not matching the connector, the sender domain is not an accepted domain, or a transport rule b…

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-mail-flow-and-connectors

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04

What is the single highest-leverage Exchange Online security setting to check first?

Whether SPF, DKIM, and DMARC are all published and DMARC is set to at least p=quarantine. Most successful phishing against a tenant either spoofs the tenant's own domain (no DMARC enforcement) or impersonates a look-ali…

Solving Microsoft 365 · www.solvingmicrosoft365.com/guides/exchange-online-mail-flow-and-connectors

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