Enhanced Status Code 5.1.3: Bad Destination Mailbox Address Syntax

Permanent failure High severity Address RFC 3463

What it means

Enhanced Status Code 5.1.3 means “Bad Destination Mailbox Address Syntax.” The recipient email address has a syntax error that makes it unparseable. The address format violates RFC 5321 rules - it may contain illegal characters, missing @ symbol, spaces, or other formatting problems.

At a glance

Code5.1.3

Bounce typeHard (permanent)

SeverityHigh

CategoryAddress

What to doSuppress the address; do not retry

StandardRFC 3463

What it looks like in your mail logs

501 5.1.3 Bad recipient address syntax (in reply to RCPT TO command)

What does 5.1.3 mean?

Enhanced status code 5.1.3 indicates the recipient address has a structural syntax problem. The server cannot even attempt to look up the address because its format is invalid according to email address standards. This is different from 5.1.1 (valid format, user does not exist) - with 5.1.3, the address itself is malformed.

Common examples include addresses with spaces, missing @ signs, double dots, trailing dots, or characters that are not allowed in email addresses. This usually indicates a data quality issue in your list where addresses were entered incorrectly or corrupted.

How 5.1.3 plays out

Your server attempts delivery

The recipient server returns a permanent 5.1.3 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 5.1.3 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
5.1.3 is		✓ this code

Common causes of 5.1.3

Email address contains spaces or illegal characters
Missing @ symbol or multiple @ symbols
Double dots (..) in the address
Address starts or ends with a dot
Special characters not properly quoted

How to fix 5.1.3

Validate email address syntax before adding to your list
Implement real-time format validation on signup forms
Clean your database of malformed addresses using regex validation
Remove the address from your list

Frequently asked questions

What does bounce code 5.1.3 "Bad address syntax" mean?

Bounce code 5.1.3 means the recipient's email address is formatted incorrectly and does not follow valid email address syntax. The receiving server rejected the message because the address violates the RFC 5321 format requirements. Common syntax errors include a missing "@" symbol, spaces in the address, invalid special characters, incomplete domain names, or malformed local parts. This is a permanent failure that requires correcting the address.

What causes a "bad recipient address syntax" error?

The error is caused by an email address that does not conform to standard format rules. Examples include "userexample.com" (missing @), "user @example.com" (space before @), "user@" (missing domain), "user@example" (incomplete domain with no TLD), or addresses with unsupported characters like commas, semicolons, or angle brackets in the wrong position. Copy-paste errors, incorrect data imports, and form submissions without validation are the most common sources of these malformed addresses.

How do I fix the 5.1.3 email address syntax error?

Review the recipient address for formatting issues: ensure the "@" symbol is present, the local part (before @) and domain (after @) are both complete, there are no spaces or illegal characters, and the domain includes a valid TLD. If you imported the address from a database or form, check for encoding issues that may have corrupted the address. Implement email address validation (using regex or a validation library) in your application to catch syntax errors before sending.

Is 5.1.3 a hard bounce?

Yes, 5.1.3 is a hard bounce because the address is structurally invalid and cannot be delivered under any circumstances without correction. Unlike a server-side issue that might resolve itself, a syntax error is a definitive problem with the address format. Remove or correct the address in your contact list immediately. If the same address consistently produces 5.1.3 errors, verify the correct address with the intended recipient through another communication channel.

Related bounce codes

Sources & further reading

RFC 3463 · The standard that defines this reply code

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