SMTP Error 551: User Not Local - Please Try Another Path

Permanent failure Medium severity Address RFC 5321

What it means

SMTP Error 551 means “User Not Local - Please Try Another Path.” The recipient is not hosted on this server and the server will not forward the message. Unlike 251 (which forwards), 551 tells the sender to retry with a different server or address, often providing the correct forwarding address.

At a glance

Code551

Bounce typeHard (permanent)

SeverityMedium

CategoryAddress

What to doSuppress the address; do not retry

StandardRFC 5321

What it looks like in your mail logs

551 User not local; please try <jdoe@relay.example.net>

What does 551 mean?

SMTP code 551 means the recipient exists but is not hosted on the server you connected to, and the server is unwilling to relay or forward the message. The server may include the correct address or server in its response, allowing you to redirect the message.

This code is relatively uncommon in modern email because most mail systems either accept and forward messages (251) or reject them outright (550). When it does occur, it often indicates a misconfigured MX record pointing to the wrong server, or an organizational migration where the mailbox has moved to a different server.

How 551 plays out

Your server attempts delivery

The recipient server returns a permanent 551 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 551 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
551 is		✓ this code

Common causes of 551

Recipient mailbox has moved to a different server
MX records point to a server that no longer hosts the mailbox
Organizational mail migration in progress
Server refuses to relay mail for the requested domain

How to fix 551

Check the bounce response for the correct forwarding address
Verify the domain MX records are pointing to the correct server
Update the recipient address in your records if a new address is provided
Contact the recipient through another channel for their current email address

Frequently asked questions

What does SMTP error 551 "User not local" mean?

SMTP error 551 means the recipient's email address is not hosted on the server you are trying to send to, and the server refuses to relay your message. Unlike the 251 response (which forwards the message), 551 is a permanent rejection indicating the server will not forward mail for unauthorized senders. This is commonly used as an anti-spam and anti-relay measure.

How do I fix SMTP error 551?

To fix SMTP 551, enable SMTP authentication in your email client's outgoing server settings by checking "My server requires authentication" and entering your credentials. Verify your SPF and DKIM records are correctly configured for your sending domain. Confirm you are using the correct SMTP server for your email account. If the error persists, check whether the recipient's server has blacklisted your IP address or domain.

What is the difference between SMTP 251 and 551?

SMTP 251 is a success response meaning "user not local, but I will forward your message." SMTP 551 is an error meaning "user not local, and I will not forward your message." The key difference is whether the server agrees to relay: 251 accepts and forwards, while 551 rejects the relay attempt. Error 551 is typically an anti-spam measure requiring the sender to authenticate or use the correct outbound server.

Related bounce codes

Sources & further reading

RFC 5321 · The standard that defines this reply code RFC 5321 Section 4.2.3: Reply Codes in Numeric Order · Defines 551 "User not local; please try <forward-path>" as a permanent reply

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