Exchange Online + On Prem email flow

Daniel Kaliel 1,241 Reputation points

We have a hybrid Online/On Prem configuration and I migrated our first account from on prem to online. Everything appears ok other than it can not receive emails from external domains. It can send/receive internal emails and it can send to external.

Microsoft Exchange Online Management
Microsoft Exchange Online Management
Microsoft Exchange Online: A Microsoft email and calendaring hosted service.Management: The act or process of organizing, handling, directing or controlling something.
4,250 questions
Microsoft Exchange Hybrid Management
Microsoft Exchange Hybrid Management
Microsoft Exchange: Microsoft messaging and collaboration software.Hybrid Management: Organizing, handling, directing or controlling hybrid deployments.
1,915 questions
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  1. Daniel Kaliel 1,241 Reputation points

    I figured it out. The issue was on the on-premise Exchange server. When an email would come in with <alias>@ssss .com the on-prem exchange server would see it as authoritative for that domain, as is should, and then attempt to route the email locally to the user. But the user isn't local anymore, it is on Exchange Online.

    I edited the users within the on-premise exchange, clicked on Email Address and changed the Set as Remote Routing Address to the Exchange Online email address <alias>@<tenant>

    Once that was done, the on premise exchange server started using the send connector created by the wizard and email began flowing.

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  1. KyleXu-MSFT 26,211 Reputation points

    @Daniel Kaliel

    550-Relaying from

    Does there exist any email filter tool, smart host or email proxy tool in your organization? In a hybrid environment, there can only exist Edge server between Exchange on-premise and Exchange online.

    So, if there exist such tool, please remove it from your organization, then remove exiting connectors(Send connectors on Exchange on-premises and Connectors on Exchange online) and rerun HCW to create new connector to routing emails in your organization.

    About the mail flow security, you could use EOP(Contained in Exchange online) to filter emails for your organization.

    By the way, make sure all need ports are opened in your organization: Exchange Online

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