Email hosting

Telstra—not Microsoft or your domain registrar—must create the reverse DNS (PTR) record for the public IP used by your Synology MailPlus server, because Telstra controls that IP address and its reverse-DNS zone.

Request the PTR record from Telstra

1. Confirm that your Telstra service has a static public IP address.
   A dynamic or private address is unsuitable for a stable PTR mapping. Telstra documents static IP availability for eligible internet services but does not publish a general self-service PTR process for ordinary static-IP add-ons.^1^
2. Choose a mail-server hostname, for example:

      mail.smcd.com.au
   

3. At your domain DNS provider, create an A record mapping that hostname to the Telstra public IP:

      mail.smcd.com.au.  A  <your-static-public-IP>
   

4. Ask Telstra to create the matching PTR record:

      <your-static-public-IP>  PTR  mail.smcd.com.au
   

   Suggested request:

   Please configure reverse DNS for my static public IP address <IP address>.
   Set its PTR record to mail.smcd.com.au.
   The corresponding A record already resolves mail.smcd.com.au to this IP.
   This is required for an SMTP server running Synology MailPlus.

5. If this is a Telstra Business, Enterprise, or Wholesale data service, check whether the service appears in CustData. Telstra’s CustData guide says eligible customers can manage reverse-DNS records in the .in-addr.arpa and ip6.arpa zones.^2^ Otherwise, contact Telstra support and ask for the request to be routed to the team responsible for static-IP reverse DNS. Microsoft cannot make this change on a Telstra-owned address.

Configure the other mail DNS records

The PTR record is separate from the DNS zone for your domain. Your domain DNS host should still hold:

• An MX record directing incoming email to mail.smcd.com.au.
• An A record directing mail.smcd.com.au to the public IP.
• One SPF TXT record authorizing that IP to send mail.
• DKIM records generated/configured by MailPlus.
• A DMARC record aligned with the domain used in the From address.

Microsoft notes that reverse DNS can be used as fallback authentication, but it is not a substitute for correctly configured SPF, DKIM, and DMARC.^3^ Microsoft 365 might reject senders whose reverse-DNS lookup fails.^4^

Also configure MailPlus so its SMTP HELO/EHLO hostname is the same hostname used by the PTR record—such as mail.smcd.com.au.

If Telstra cannot provide PTR service

Do not create a PTR record in your ordinary domain DNS panel; that does not control reverse DNS for Telstra’s IP. The practical alternatives are:

• Use an SMTP relay provider that supplies suitable reverse DNS.
• Host outbound mail with Microsoft 365/Exchange Online or another mail provider.
• Move to a business internet service that supports static IP and reverse DNS.

If you instead move all email hosting to Microsoft 365, its domain setup uses MX, SPF, Autodiscover, DKIM, and related records; Microsoft operates the reverse DNS for its own outbound servers.^5^

References

1. What Is an IP Address (Dynamic and Static)? | Telstra
2. CUSTDATA USER GUIDE - Telstra
3. Security Operations guide for email authentication in Microsoft 365
4. External senders - Troubleshoot email sent to Microsoft 365
5. External Domain Name System records for Microsoft 365