553 5.1.7: The sender's address is not valid syntax

At a glance

• Type: permanent failure (class 5, RFC 3463 §2). The sending server does not retry; sending the same message again fails the same way until the cause is fixed.
• Basic code 553: “Requested action not taken: mailbox name not allowed (e.g., mailbox syntax incorrect)” (RFC 5321 §4.2.3).
• Enhanced code 5.1.7: “Bad sender's mailbox address syntax” in the IANA registry (RFC 3463), in the group addressing status (RFC 3463 §3.2).

The receiving server rejected your sender address, not the recipient. The address you gave in MAIL FROM is not valid syntax, so this message cannot be delivered to anyone at that server until you fix your own setup.

What it means

RFC 3463 §3.2 defines X.1.7 as bad sender's mailbox address syntax: the sender's address was syntactically invalid, in any part. The basic code 553 says the mailbox name is not allowed (RFC 5321 §4.2.3).

The address in question is the envelope sender, the one your system sends in the MAIL FROM: command. It is often different from the From header your readers see. Bounces go back to it, and receivers usually record it as the Return-Path. The syntax rules are in RFC 5321 §4.1.2.

An empty sender, written MAIL FROM:<>, is legal. RFC 5321 §4.5.5 requires it for bounce messages and other delivery notifications, so a null sender on its own is not a syntax error.

Common causes

• The application puts a display name into the envelope, such as MAIL FROM:Support <help@example.com>.
• A server or script sends as a bare user name, or as root@localhost, because no sender domain was configured.
• A variable in a template was empty, leaving something like @example.com.
• Spaces, quotes or a trailing dot ended up in the configured address.
• A domain label contains an underscore, which RFC 5321 §4.1.2 does not allow.

How providers use it

Gmail returns 553 5.1.7 when the sender address is invalid (Google's SMTP error reference). When the sender address is well formed but its domain cannot receive mail, receivers use other codes, such as 5.1.8 or, for a domain with a null MX, 550 5.7.27.

How to fix it

1. Find where your system sets the envelope sender. Apps call it the return path, bounce address or envelope from.
2. Set it to a complete address at a domain you control, such as bounces@example.com.
3. Check the raw SMTP log for the exact MAIL FROM: line your system sent. The fault is often visible there and not on the app's settings screen.
4. If you use a mail library, pass the sender as a plain address and the display name separately.
5. Send a test to a mailbox you own and check the Return-Path of the received message.

Related codes

• 5.1.8: The sender's domain is refused
• 553 5.1.3: The recipient address is not valid syntax
• 503 5.5.1: Commands sent in the wrong order

See every bounce code in the dictionary.

Many bounces from addresses that do not exist can be avoided: check an address with our free email checker, or clean a whole list before you send.

How we wrote this: drafted by the email-validation.org team with the help of AI writing tools, from our product documentation and the sources linked in the text.