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.
What Does Error 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.
Common Causes
- 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 Error 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
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.
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 Clear your email client's auto-complete cache, as it may contain an outdated version of the address.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.
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.