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Mail Flow in Office 365

Updated: May 21

When it comes to emails, emails are the most important aspect of any productivity or business, and they should always be set 24/7. We always expect that there will be minimal downtime.

Different providers provide different levels of security when it comes to emails, and all of the providers have a basic expectation that there is a smaller number of vulnerabilities, spam, spoof or account compromised issues, due to which they invest much focus so that there is no security breach.

As there are many mail spam filtering service providers like Mimecast, Proofpoint, Barracuda, Symantec and many more including Office 365 which provides email security.

The email protection services provided by Office 365 are called Exchange Online Protection or EOP has their own architecture, and no matter where you point the Filtering Service, it has to come in or go out by reaching Office 365 as the mailboxes are hosted in Cloud Office 365.

Likewise, we have sent and receive connectors in Exchange On-premise, we have inbound connectors and outbound connectors in Office 365 and all other Filtering providers too. Now, what are they specifically called by other Filtering providers? That is a different story. Some may call it send/receive connector inbound/outbound or something else.

What Are Connectors and What Can They Do?

Connectors are a set of instructions that customize the way your email flows. It controls the flow of email to and from your Office 365 organization. Here’s why you need to set up connectors:

  • Enable mail flow between Office 365 and your on-premises Exchange environment.

  • Apply security restrictions or controls to mail exchanges with a business partner or service provider.

  • Enable email notifications from a printer or other non-mailbox entity.

How to Configure Connectors?

In order to route emails correctly, you will need to configure two connectors in Office 365. One connector must route emails from Office to your on-premises Exchange Server, and a second connector must route emails from your on-premises Exchange Server to Office 365. Let’s further discuss these two connectors and how they route emails.

1) Office 365 to On-Premises Exchange Server

After Office 365 subscription and configuration, you need to set up Office 365 to accept all emails on behalf of your organization. For this purpose, you will need to configure connectors and then point your domain’s MX (mail exchanger) record to Office 365. To prepare for this scenario, you must create a send connector in the form of smart hosts in your on-premises Exchange environment.

To create a connector in Office 365, click Admin, and then click Exchange to go to the Exchange admin centre. Next, click Mail flow and click Connectors. Click Add, and follow the instructions in the wizard to create a connector from Office 365 to your on-premises Exchange servers.

Office 365 to On-Premises exchange server

2) From On-Premises Exchange Servers to Office 365

With the help of this connector, Office 365 will accept all messages from your organization’s email servers and send the messages on your behalf to recipients in Office 365 as well as to external recipients like Gmail. Now, in order to complete this scenario, you will further need to configure your Exchange servers to send email messages directly to Office 365.

For example, to create a new Send connector in your on-premises Exchange server, you will need to enter the following information:

Name: OnPremse to Office 365 FQDN: SmartHosts:

In order to create a connector in Office 365 portal, click Admin, then click Exchange, and then go to the Exchange admin center. Next, click Mail Flow and then click Connectors. Click Add, and follow the instructions in the wizard to create a connector from your on-premises Exchange servers to Office 365.

Mail flow in Office 365

After completing the Wizards in both connectors, you will find two connectors created in Office 365 connectors, as below.

exchange admin center

There are two reasons why you need to set up Office 365 connectors.

  • The first reason is if you have a standalone Exchange Online Protection (EOP) subscription scenario, which means you have your own email servers (also called on-premises servers) and you subscribe to EOP only for email protection services against spam filtering and malware protection.

  • The second reason is that if you have some mailboxes in the Office 365 environment and some mailboxes in the On-premises Exchange Server environment, you want to enable mail flow between these two environments. Before you set up connectors for these two scenarios, you must make sure whether you only need connectors or if an Exchange hybrid deployment better meets your business needs.

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