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Using EOP for all inbound hybrid email

September 27, 2013

In a hybrid environment, you can change your external MX records to deliver all inbound Internet email through EOP. Email will be accepted and delivered for both online and on-premise mailboxes. However, all AD user accounts with on-premise mailboxes must be synchronized to MSOL otherwise EOP will generate an NDR:

#< #5.1.1 smtp;550 5.1.1 RESOLVER.ADR.RecipNotFound; not found> #SMTP#

Hybrid mail-flow uses a service domain namespace for cross-premise mail delivery, not an Internal Relay accepted domain. Exchange Online must therefore be populated with details of all on-premise mailboxes.

If you have applied filters to DirSync (FIM/MIIS) then make sure you don’t exclude any on-premise accounts that require inbound external SMTP email. Remember that system critical accounts, such as domain Administrator, are automatically excluded from DirSync.

Although it is physically possible to use EOP for all inbound hybrid email, ensure you don’t contravene any service or license agreements in doing so

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