SMTP RELAY for OFFICE 365 – Configure, Test, and Troubleshoot

In this article, we’ll cover how to configure, test and Troubleshoot SMTP Relay in Office 365.

What is SMTP?

SMTP is a standard language (protocol) used by all the applications on this planet to send emails.

What is SMTP Relay?

SMTP Relay is a service, which accepts email from the email sender program and forwards it to the email receiver server or another node.

What is Office 365?

Office 365 is a subscription-based service that ensures you always have the most up-to-date modern productivity applications like Word, Excel, Outlook, PowerPoint, Publisher, OneNote, OneDrive, SharePoint, Exchange, Microsoft Teams, Skype for Business, Power BI, Kaizala, PowerApps, etc from Microsoft.

Basic requirements for SMTP Relay with Office 365
  • The sending app must connect to the Office 365 servers on port 587
  • The application which you are using for sending the emails must support TLS
  • The sending app must authenticate with Office 365
  • The account you authenticate to the relay server must be the same account as the from the address on the messages you send through the relay.
Setting Up Office 365 Connector for Mail Flow

We need to set up connectors which is nothing but the set of instructions that customize the way your email flows. It regulates the flow of email to and from your Office 365 organization.

There are mainly two scenarios for which you required to set up connectors to route emails between Office 365 and on-premises Exchange servers.

#First: Your mailboxes in your on-premises Exchange servers and you want to use Exchange Online Protection for protecting email from spam filtering and malware protection.

#Second: You have mailboxes in both Office 365 and on your on-premises Exchange servers. And you want to enable mail flow between these two environments.

The below image shows how Office 365 connector works with your on-premises Exchange Servers in both scenarios.

SMTP Relay connector

In the above image, there are two Sam and Lisa employees from the same company. Lisa has a mailbox which is located in Office 365 and Sam has a mailbox in Exchange Server that you manage.

They have a client name, Emma. And, both Sam and Lisa exchange emails with Emma. Emma’s email address is from Gmail. Now:

  • While an email is sent to Sam and Lisa, connectors are required.
  • Likewise, when an email is sent to Sam and Emma, connectors are required.
  • And when an email is sent to Lisa and Emma, no connector is required.

Therefore, if you own your on-premises Exchange Server and Office 365 subscription, you must be obliged to set up connectors. Without connectors, you will not be ready to send or receive emails.

How to Configure Connectors?

You need to configure two connectors in order to route the emails correctly.

  1. One connector will route emails from Office 365 to your on-premises Exchange Server
  2. And, the second connector will route emails from on-premises Exchange Server to Office 365.


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