Add mail subdomain question in Office 365

Question

Hey All,

I have a question about subdomain email and Office365.

Consider the following:

• Primary Custom O365 domain: contoso.com.
• No DNS or Office365 configurations for @Sales .contoso.com
• Hybrid exchange

These are my requirements:

1. To send email from my in-house app as support@Sales .contoso.com
2. The reply-to address to be support@Company portal .com so customers can hit reply on the email and the email goes to support@Company portal .com (mailbox exists in @Company portal .com)
3. Any NDR from email sent from the support@Sales .contoso.com goes back to support@Sales .contoso.com mailbox.

Please help with how I set this up?

1. Do I need DNS MX records?
2. Is there a need for a new subdomain in Office365?
3. Can the mail address support@Sales .contoso.com be just an alias on the @Company portal .com domain and be added to support@Company portal .com mailbox?

These are my general reqs but happy to tweak if needed.

I really appreciate your attention and support.

Thanks, M

Answer 1

Hi @SenhorDolas

Yes, you need to add the MX record after you create the subdomain.

According to my research, we could associate a mailbox with the Microsoft Exchange Recipient to copy internal NDRs.
Check this document, it might be helpful for you.

https://learn.microsoft.com/en-us/previous-versions/office/exchange-server-2007/bb400930(v=exchg.80)#copying-internal-ndrs-to-a-mailbox

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Answer 2

So, this is a two part answer, although if your exchange is on-prem not in the cloud, please disregard this.

1. You can add a Subdomain to your tenant, read more.
   This will connect the subdomain to your tenant, you would also (most likely) need a Exchange Plan for the shared mailbox.
2. The second part requires a bit of Powershell and is described here.

I hope this helped.

Answer 3

Hi @SenhorDolas ,

Based on your distribution, maybe mailflow rule could help you to work around.
To send email from my in-house app as support@Sales .contoso.comThe reply-to address to be support@Company portal .com so customers can hit reply on the email and the email goes to support@Company portal .com (mailbox exists in @Company portal .com)

1.Open EAC and go to mail flow rule.

2.You could follow below screenshot to create a mailflow rule to auto forward email to support@Company portal .com when recipient is support@Company portal .com.

According to my test, it could not be able to send to another mailbox automatically. Because NDR is sent by the Exchange system not by the email account.
About NDR
When there's a problem delivering a message, Exchange sends an NDR to the message sender that indicates there was a problem. NDRs include a code that indicates why the message wasn't delivered, and possible solutions to help get the message delivered.

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Answer 4

@Aaron Xue-MSFT + @Daniel Kåven

Hi Everyone, sorry for the delay coming back.
You replies have helped me get up and running but I still have more questions... :)

I can see from the docu that I can add the subdomain and it will validate automatically since it's a subdomain: domains-faq

- Will I still need to add an MX record on my DNS registrar?

- After I add the subdomain to Office365 only, how will my on-prem Exchange know about this new subdomain? Are there any changes needed on AD Connect or on AD?

- Since I don't want this email domain assigned to any other mailbox, should I edit the ;Email Address Policy; on exchange on-prem?

It think one way to get away of creating a mailbox for this subdomain is to add the alias to another mailbox with the main domain, so in my case add the alias: support@Sales .contoso.com to sales@Company portal .com mailbox so all replies and NDRs come back to an existing mailbox. Will this work OK?

Many thanks for your time and interest on this post. Really appreciate the help.

Thanks, M