How to resolve email errors 550 5.1.351 and 550 5.7.1?

Hi Jeffrey,

Thank you for reaching out.

Because the issue started after moving the domain away from GoDaddy, this likely involves an outdated Proofpoint configuration rather than Outlook itself. The 550 5.1.351 error means the destination server cannot validate the mailbox, while 550 5.7.1 usually indicates that a security gateway or mail-flow policy rejected the message.

To resolve the issue, please follow these steps:

1. Check the rejection details

Open the non-delivery report and look for “Message rejected by.” If it lists a server such as mx1-us1.ppe-hosted.com, the rejection is coming from Proofpoint. Similar cases have occurred when Proofpoint continued treating an address as invalid after a mail-service change.

2. Verify the domain’s DNS records

At GoDaddy, confirm that:

• The MX record points to your current Microsoft 365 mail server.
• Old Proofpoint MX records have been removed if Proofpoint is no longer being used.
• The SPF record authorizes Microsoft 365 and does not contain an obsolete Proofpoint configuration.
• The affected domain is verified and configured as an accepted domain in the Microsoft 365 admin center.

3. Check for a Proofpoint connection

Please check the domain’s DNS records directly for any remaining Proofpoint connections, particularly MX records referencing ppe-hosted.com. If this value is still present, incoming email may continue to route through Proofpoint before reaching Microsoft 365. Please also review the other DNS records for any Proofpoint-related entries, as these may indicate that the domain is still connected to the previous email-security configuration.

I hope your issue gets resolved soon. Any updates would be greatly appreciated.