Enhanced Status Code 5.1.4: Destination Mailbox Address Ambiguous

Permanent failure Medium severity Address RFC 3463

What it means

Enhanced Status Code 5.1.4 means “Destination Mailbox Address Ambiguous.” The recipient address matches multiple mailboxes and the server cannot determine which one to deliver to. This is rare and typically occurs with misconfigured mail routing or directory services.

At a glance

Code5.1.4

Bounce typeHard (permanent)

SeverityMedium

CategoryAddress

What to doSuppress the address; do not retry

StandardRFC 3463

What it looks like in your mail logs

Remote Server returned '550 5.1.4 RESOLVER.ADR.Ambiguous; ambiguous address'

What does 5.1.4 mean?

Enhanced status code 5.1.4 indicates the address resolves to multiple possible mailboxes and the server cannot determine the correct one. This is uncommon in modern email systems but can occur in complex enterprise environments with overlapping mail routing rules or directory aliases.

This error typically requires the recipient organization to fix their mail routing configuration. As a sender, there is little you can do except contact the recipient through an alternative channel.

How 5.1.4 plays out

Your server attempts delivery

The recipient server returns a permanent 5.1.4 rejection

This is a hard bounce: the message will not be accepted as sent

Suppress the address and fix the root cause before resending

Where 5.1.4 sits: soft vs hard bounce

	Soft bounce (4xx)	Hard bounce (5xx)
Nature	Temporary	Permanent
SMTP class	`4xx`	`5xx`
What to do	Let it retry	Suppress the address
Recoverable?	Often	No
5.1.4 is		✓ this code

Common causes of 5.1.4

Address matches multiple mailboxes in the directory
Overlapping mail routing rules create ambiguity
Distribution list and individual mailbox name conflict

How to fix 5.1.4

Contact the recipient through an alternative channel
Ask the recipient organization to resolve the ambiguity in their mail configuration
Try a more specific email address if available

Frequently asked questions

What does bounce code 5.1.4 "Destination mailbox address ambiguous" mean?

Bounce code 5.1.4 means the email address you specified matches multiple recipients on the destination system, and the server cannot determine which mailbox to deliver to. This ambiguity prevents delivery because the server will not guess the intended recipient. The address needs to be corrected to uniquely identify a single mailbox before the email can be delivered.

How do I fix a 5.1.4 ambiguous address bounce?

Double-check the recipient's email address for typos, extra characters, or copy-paste errors that may have introduced invisible characters. Contact the recipient to confirm the exact, complete email address. If the address contains common name patterns (like "john@" on a server with multiple Johns), use the full, unique address provided by the organization. This is a permanent error that requires manual correction before resending.

What causes a "destination mailbox ambiguous" error?

This error occurs when the recipient's mail system finds that the address you specified matches more than one mailbox. Common causes include partial addresses, distribution group name conflicts, alias overlaps in the organization's email directory, or addresses created by copying text that includes hidden formatting characters. The receiving server refuses to guess which recipient you intended, so it rejects the message and asks you to clarify.

Related bounce codes

Sources & further reading

RFC 3463 · The standard that defines this reply code Microsoft Learn, Fix NDR error 550 5.1.1 through 5.1.20 in Exchange Online · Exchange Online troubleshooting for the 5.1.x address-resolution NDR family.

Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes