Hello All
Welcome back to the Exchange Hybrid deployment Series - Part 3. In this part, we are going to see, what is centralised mail flow and different scenarios of Email Flow in Exchange Hybrid 2016 environment
what is centralised mail flow?
centralised mail flow is a feature of Exchange Hybrid deployment that lets an organisation to manage the email routing from on premise Exchange Server.
Email Flow in Exchange Hybrid 2016 environment
- first scenario is your MX record is pointing to EOP and centralised mail flow is enabled.
- Second scenario will be MX record is pointing to EOP and centralised mail flow is disabled.
- Third scenario will be what will be the mail flow when MX record is pointing to on premise.
- Fourth scenario will be how outbound emails will be routed to internet when centralised mail flow is enabled.
- Fifth scenario will be the email flow of outbound emails when centralised mail flow is disabled.
first let's understand what is centralised mail flow in Exchange Hybrid.
Centralised mail flow is a feature of Exchange Hybrid deployment that lets an organisation to manage the email routing from on premise Exchange Server.
When we run it HCW we can choose if we want to enable centralised mail flow when you enable centralised mail flow.
The HCW enables your on premise Exchange Server to function as a smart host
. That means now all the inbound and outbound emails will be managed by on premise Exchange Server.
Let's assume that we have on premise exchange and Exchange Online and we have deployed hybrid.
Now we have pointed MX record to office 365. That means any email that is sent from internet will be delivered to office 365.
EOP will scan that email and then it will be delivered to the users.
But my requirement is whenever my organisation received any email from outside world or a user sends an email to external organisation, that email should be processed by an on premise Exchange server and then it should be delivered to the recipients
let's say your requirement is that whenever an email is coming to my organisation or it is going out of my organisation, a compliance rule or a disclaimer should be attached to that email in on premise exchange and then it should be further processed.
So this is where we use centralised mail flow in Exchange Hybrid where every external, incoming or outgoing email is processed by an on premise Exchange server and then it is delivered to the recipients
when we run Hybrid Configuration Wizard it creates two connectors in Exchange Online inbound and outbound when we enable centralised mail flow in HCW
It sets route all messages via an on premise attribute to true .
if you run get-outboundonnector | fl in Exchange Online you will see this output if route all messages via on premise attribute is set to true
That means centralised mail flow is enabled in your Exchange Hybrid environment.
So when you enable centralised mail flow in HCW it sets route all messages via an on premise attribute to true if this value is set to false That means centralised mail flow is disabled
The first scenario that we are going to discuss is how inbound emails will be routed in Exchange Hybrid deployment when we have MX record pointed to EOP and centralised mail flow is enabled.
In this example, we have one user mailbox in on premise exchange with named onprem1 and one user mailbox in Exchange Online with named aishabhari.
Someone from outside has sent an email to onprem and aishabhari and Both users are using cloudmonkeys.xyz domain in their email address.
So the email will be delivered to EOP because MX record for this domain is pointing to EOP as soon As this email will reach Exchange Online Protection, EOP will scan this email. And because centralised mail flow is enabled, after scanning the email, EOP will route the email to on premise Exchange Server.
Then on premise Exchange Server will perform a lookup for each recipient. Through the lookup, it will determine that onprem1 mailbox is in on premise. And Aisha mailbox is in Exchange Online.
So Exchange Server will split the email into copies because we have two recipients for the same email. One copy of the email will be delivered to John's mailbox and the second copy of the email will be sent back to EOP. Then EOP will send that email to Exchange Online and Exchange Online will deliver this email to Aisha's mailbox.
So this is how email flow will work when MX is pointing to Exchange Online and centralised mail flow is enabled.
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The second scenario that we are going to discuss is what will be the mail flow for inbound emails in Exchange Hybrid deployment when MX record is pointing to EOP and centralised mail flow is disabled.
So in this particular scenario, we have centralised mail flow disabled. That means we are using the Decentralised mail flow and MX record is pointing to EOP.
So let's assume we have exchange hybrid deployed. We have one user in on premise with named onprem2, and one user is in Exchange Online with them alanreid.
Someone from Internet has sent an email to onprem2 and alanreid. Both users are using same domain cloudmonkeys.xyz and MX record for this domain is pointing to EOP.
The email will be delivered to Exchange Online Protection. EOP will scan the email for viruses and will send this email to Exchange Online
Exchange Online will do recipient lookup and will find that onprem2 mailbox is located in on premise and alanareid mailbox is located in Exchange Online.
Exchange Online will split this email into two copies. One email will be delivered to Alanreid mailbox and Exchange Online will send the other copy that to EOP. Then EOP will send the other copy of the email to on premise Exchange Server
On Premise Exchange server will send this copy to mailbox server and email will be delivered to onprem2 mailbox.
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The third scenario of Exchange Hybrid is when MX record is pointed to On premise, so how emails will flow in Exchange Hybrid deployment.
In this scenario, we have one user mailbox in on premise with name onprem3 and one mailbox in Exchange Online with named Ramki.
If someone from Internet has sent an email to onprem3 and ramki. Both users are using same domain for their email addresses [cloudmonkeys.xyz].
Email will be routed to the on premise Exchange server because MX is pointing to on premise.
The on premises Exchange Server will perform recipient lookup using global catalogue server.
On Premise server will find that onprem3 mailbox is located in on premise and ramki mailbox is located in Exchange Online.
Because on premise server will find a remote routing address for ramki which is ramki@cloudlmonkeys.mail.on microsoft.com.
So with the help of this address on premise server will come to know that this is a remote mailbox, or this user exists in office 365.
On Premise Exchange Server will split the email into copies.
one copy of the email will be sent to on premise Exchange mailbox server and it will be delivered to onprem3 mailbox and exchange server will send the other copy to EOP through the send connector that is configured .
EOP will scan the email for viruses and it will send this email to Exchange Online. Then Exchange Online will deliver this email to ramki's mailbox. So this is how inbound emails flow work
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Now let's understand how outbound mail flow work in Exchange Hybrid deployment, when centralised mail flow is enabled.
Fourth Scenario:
We have a user in Exchange Online with named alanaisha. He is going to send an email to an external user. As soon as alanaisha will send email from his email application or outlook , the email will go to EOP.
Exchange Online Protection will scan the email and will route this email to on premise Exchange Server using TLS.
On Premises Exchange Server will scan this email for viruses and will perform other checks those are configured by administrators in on premise for example, transport rules or compliance rules or a disclaimer
Then on premises Exchange Server will look for the MX record for the external domain and it will send the email to the external domains email server, and this is how the email will be delivered to the external user.
Fifth Scenario :
The fifth scenario that we are going to discuss is how outbound emails flow will work in Exchange Hybrid deployment when centralised mail flow is disabled. [Decentralised Model]
we have a user in Exchange Online with named alanaisha and he is going to send an email to external user. As soon as alanaisha will send email from his email application or outlook, the email will be routed to Exchange Online action or EOP.
Exchange Online Protection will scan this email and it will perform MX record lookup for external users domain and EOP will send this email to the email server of external domain.
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