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Fix Incoming Emails Bouncing Back: Mailbox Does Not Exist Error
Question details
Users are experiencing intermittent bounce backs for incoming emails on a custom domain, with errors stating the mailbox does not exist.
- Product
- Email Service / Mail Server
- Device & OS
- not provided
- Scenario
- Receiving emails from external senders on a custom domain.
- Observed behavior
- Senders receive a non-delivery report (NDR) indicating the recipient's mailbox does not exist, even though the mailbox is active.
Verify that the sender has spelled your email address correctly, and ensure that your mailbox is active, licensed, and not exceeding its storage quota.
Verify Domain MX and DNS Records
Check your domain's DNS settings to ensure mail is properly routed to your correct email servers.
Incorrect MX (Mail Exchanger) records are the most common cause of domain-level routing failures, causing the internet to look for your mailbox on the wrong server.
Access the dashboard of the company where you purchased your custom domain (e.g., GoDaddy, Namecheap, Cloudflare).
Locate the DNS Management or Advanced Zone Editor section for your domain.
Check your MX records to ensure they point exactly to your current email hosting provider's required server addresses. Remove any old or conflicting MX records.
Save your changes. Keep in mind that DNS updates can take anywhere from 1 to 48 hours to fully propagate globally.
Analyze the Complete Bounce Message (NDR)
Obtain the Non-Delivery Report from the sender to identify specific server-side rejections and diagnostic codes.
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Frequently Asked Questions
What is an MX record and why does it cause bounce backs?
An MX (Mail Exchanger) record is a DNS setting that directs incoming emails to your designated mail server. If it is missing or points to an outdated server, incoming emails cannot reach your inbox and will bounce back to the sender.
Why does the bounce error say 'mailbox does not exist' if I can still log in and send emails?
This usually happens when a sender's email client routes the message to an old, deactivated server or when your custom domain's DNS is misconfigured. Your login works on the correct server, but the internet is trying to deliver mail to the wrong location.
Can incorrect SPF or DKIM records cause mailbox not found errors?
Yes. If your domain's SPF or DKIM records are strictly configured but incorrect, the receiving mail server might reject the message during routing to prevent spoofing, sometimes generating a generic 550 or mailbox unavailable error.
How do I trace missing emails if my mailbox is hosted in Exchange Online?
If you are an administrator for Microsoft 365 or Exchange Online, you can use the Message Trace tool in the Exchange Admin Center. This tool tracks incoming messages and shows exactly where and why they were dropped, deferred, or rejected.
