Enhanced Status Code 5.1.0: Other Address Status

Permanent failure High severity Address RFC 3463

What it means

Enhanced Status Code 5.1.0 means “Other Address Status.” A general address-related permanent failure. The recipient address has a problem that does not fit a more specific category. This often indicates the address is invalid or the domain cannot be reached for delivery.

At a glance

Code5.1.0

Bounce typeHard (permanent)

SeverityHigh

CategoryAddress

What to doSuppress the address; do not retry

StandardRFC 3463

What it looks like in your mail logs

550 5.1.0 Address rejected user@example.com

What does 5.1.0 mean?

Enhanced status code 5.1.0 is a general addressing error used when the server determines there is a permanent problem with the recipient address but cannot classify it more specifically. It often appears when the domain exists but some aspect of the address resolution fails permanently.

Like 5.0.0, this is somewhat generic. The more specific codes 5.1.1 (user unknown), 5.1.2 (bad domain), and 5.1.3 (bad syntax) are preferred. When you see 5.1.0, read the accompanying message text for details.

How 5.1.0 plays out

Your server attempts delivery

The recipient server returns a permanent 5.1.0 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 5.1.0 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
5.1.0 is		✓ this code

Common causes of 5.1.0

Address-related failure that does not match a specific sub-code
Domain resolves but delivery fails for an unspecified address reason
Mail routing configuration issue for the recipient domain

How to fix 5.1.0

Verify the recipient email address is correctly formatted
Check that the recipient domain has valid MX records
Read the full bounce text for specific details about the address failure

Frequently asked questions

What does bounce code 5.1.0 mean?

Bounce code 5.1.0 means "Other address status": there is a problem with the recipient's email address that caused the delivery failure, but the exact nature of the problem is not specified beyond an addressing issue. Per RFC 3463, codes with X.1.0 indicate "something about the address specified in the message caused this DSN." This is a permanent failure requiring address correction.

How do I fix a 5.1.0 "Address rejected" bounce?

First, carefully check the recipient's email address for misspellings, missing characters, or incorrect domain names. Clear your email client's auto-complete cache, as it may contain an outdated version of the address. If the address appears correct, contact the recipient directly to confirm they have not changed addresses or blocked your sender. Also check whether your domain or IP has been blacklisted by the recipient's mail server.

What causes a 5.1.0 bounce in Exchange Online?

In Exchange Online (Microsoft 365), a 550 5.1.0 bounce commonly occurs when the recipient's address has changed (such as moving from on-premises Exchange to Exchange Online), your email address is blocked by the recipient, or your auto-complete list contains an invalid cached entry. To resolve it, remove the recipient from your auto-complete cache, re-type the address manually, and verify the recipient has not configured a mail flow rule that blocks your messages.

Related bounce codes

Sources & further reading

RFC 3463 · The standard that defines this reply code RFC 3463: Enhanced Mail System Status Codes · Defines X.1.0 "Other address status" for general addressing failures

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