Enable SMTP Port 587 instead of Port 25

Through my Office365 subscription, I have an SMTP relay, ‘contoso-com.mail.outlook.com’. Currently this only accepts traffic on port 25

I want to look at setting up port 587 with TLS encryption, but keep port 25 running in junction with port 587 for the time being on the same SMTP relay.

My problem is that I'm not sure where my current physical mail server is (if it exists), I wasn't the one who originally set this up. Running nslookup only lists my Active Directory server

I currently have ‘contoso-com.mail.outlook.com’ listed under my domains as an MX record. As well as 'contoso.com' connector running with Microsoft Exchange admin center.

Mail flow settings for connector:

Mail flow scenario

From: Your organization's email server

To: Office 365

Not much experience with SMTP relays so apologies for that

You should be able to use smtp.office365.com on port 587 for outgoing e-mail.

What's listed as MX for your domain is for receiving e-mail. Microsoft supports StartTLS on port 25 for their Exchange 365 service. All other MTA's in the world will contact the MX at port 25, so there's no point in having port 587 enabled on a MX. That's only used for outgoing mail from client devices.

The fact that they support StartTLS can be verified with the command openssl s_client -starttls smtp -crlf -connect hostname:25 which will output the certificate the host replied with.

Office365 is a hosted solution, where Microsoft runs your mail service. You don't pay for the server; you pay for sending and receiving e-mail as a service, and don't have to care about patching, upgrading or any configuration. That's simply handled by Microsoft. You have to take what you get; you can't modify Microsofts server environment, as it's shared with other customers.

Can you send a message out via the current setup and then examine the headers? They should show the entire flow of mail through the system and give you an idea of where to start looking.