How to Send Emails from Third Party apps using Microsoft Exchange Mailbox SMTP

by Jack
5 minutes
How to Send Emails from Third Party apps using Microsoft Exchange Mailbox SMTP

New migrants to Microsoft Exchange are now suffering from this problem where they are not able to send mails from their mailbox over SMTP. The outbound emails are blocked due to the lack of STARTLS protocol in their third-party app. Be it a CRM or some other internal app, in order to send mails over SMTP from third party app you have to turn on a legacy setting from Microsoft Exchange Admin Center. The default SMTP settings that do not work in 3rd party apps for new Microsoft Exchange mailboxes are:

  • SMTP Server: and any other regional SMTP server of Office 365.
  • SMTP Port: 587
  • Security Protocol: SSL & TLS

If you try the above settings in third party app that saves logs, then you will most likely get the following error.

"5.7.3 STARTTLS is required to send mail [239.INDPRD.PROD.OUTLOOK.COM]"

Fix "STARTTLS is required to send mail " Error  to Send Emails over SMTP from Microsoft Exchange?

There are two ways to send emails over SMTP though Microsoft Exchange server:

  1. Use Legacy SMTP Server
  2. Use SMTP Relay.

The first option is easy and probably exactly what you want. Basically, you can configure your existing mailbox to be used to send emails over SMTP from third party apps. For this, you will have to turn on a simple setting. Follow the steps below to do it.

  1. Log into EAC if you are administrator of Exchange Server in your organization.
  2. Go to Settings from the sidebar.
    Exchange Admin Center Settings
  3. Click on "Mail flow" and Mail flow settings popup that appears, check Turn on use of legacy TLS Clients option.
    Exchange Admin Center Settings Mail flow

Now, to send emails over SMTP using your Microsoft Exchange or Outlook account, use these settings instead.

  • SMTP Server:
  • Port: 587
  • Security protocol: Not really required. But you can try using TLS.

The second option is mostly suited for 3rd party apps but you will have to setup a relay from EAC (Exchange Admin Center). This can be a long and tedious process as it requires changing MX records in your DNS hosting provider. If you are okay in setting up an SMTP relay (send only account) then you can follow the process on official Microsoft Docs.

office 365 docs to set up SMTP Relay


In this guide, you have learnt a way to get away with the SMTP problem that you face in Microsoft Exchange mailboxes. There are 2 methods that you can consider. Actually, there is 3rd method as well which is called "Direct Send" and I would urge you to check that out on Microsoft Docs as well.