Fix Email Delivery Between Domains on Different Providers
Question details
The user needs to resolve email delivery failures that occur when sending messages between domains that are hosted on different providers, due to conflicting DNS configurations.
- Product
- Microsoft 365 / DNS
- Device & OS
- not provided
- Scenario
- Managing email domains where one domain is hosted on Microsoft 365 and another remains on an external VPS, or piloting Microsoft 365 with split delivery.
- Observed behavior
- Email delivery fails because conflicting MX records confuse the sending server, preventing the message from routing to the correct external VPS or Microsoft 365 mailbox.
Before modifying your DNS settings, ensure you have administrative access to your domain registrar or DNS hosting provider, and verify exactly which provider is intended to host your active mailboxes.
Update MX Records to Point to a Single Email Provider
Prevent mail routing conflicts by ensuring your domain's MX records point exclusively to the actual active email host.
Having duplicate or conflicting MX records for a single domain (e.g., pointing to both Microsoft 365 and a VPS) causes unpredictable mail routing. Mail servers may attempt to deliver messages to the wrong platform, resulting in bounces or lost emails.
Log in to the administrative portal of your DNS hosting provider (such as GoDaddy, Cloudflare, or your VPS control panel) and navigate to the DNS Management or Zone Editor section.
Search your domain's DNS zone for existing MX (Mail Exchanger) records. You will likely see multiple entries if your domain is split between providers.
Delete any MX records that belong to the inactive provider. Ensure that the only remaining MX records are those provided by the service actively hosting your email (e.g., solely Microsoft 365 or solely your external VPS).
Save your changes. DNS updates can take anywhere from a few hours to 48 hours to fully propagate across the global internet.
Configure Internal Relay for Microsoft 365 Pilot
If you are intentionally splitting mailboxes across two providers (piloting), configure the domain as an Internal Relay in Microsoft 365.
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Frequently Asked Questions
Why are emails failing between two domains I own on Microsoft 365?
This usually happens if one domain is verified in Microsoft 365 but its mailboxes are actually hosted elsewhere (like an external VPS). Microsoft 365 attempts to route the message internally instead of checking public DNS, which causes the delivery to fail if the mailbox isn't physically on Microsoft's servers.
Can I use both Microsoft 365 and another email provider for the same domain?
Yes, through a setup called split delivery or piloting. You must configure your primary host to accept all mail and automatically route messages for unknown users to the secondary host via specific mail flow connectors, rather than using duplicate MX records.
How long does it take for MX record updates to fix my email routing?
Standard DNS propagation can take anywhere from a few hours to 48 hours. During this period, some internet service providers might still use the old, cached MX records, leading to intermittent delivery failures until the cache clears.
