503: bad sequence of commands
The client sent a command at the wrong point in the conversation, for example message data before any recipient was accepted, or mail before logging in.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 503
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
503 Bad sequence of commands
What it means
SMTP has a fixed order: greeting, EHLO, optional STARTTLS and AUTH, MAIL FROM, one or more RCPT TO, then DATA. A 503 says a command arrived out of turn. A common case is DATA after every recipient was refused, and RFC 5321 also tells a server that refused the session at connection time to answer any later command with 503. Some servers send 503 when a client tries to send mail before authenticating. It is permanent for that command; the enhanced code (5.5.1, or 5.7.0 when login is missing) narrows it down.
Common causes
- DATA sent after all recipients were refused.
- MAIL FROM sent before EHLO or HELO.
- An attempt to send before logging in, on a server that requires authentication.
How to fix it
- 01Look at the earlier replies in the session: a refused RCPT TO usually comes just before the 503.
- 02If the server requires it, make sure your client authenticates before sending.
- 03Fix or update sending software that issues commands out of order.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- RFC 5321, section 4.2.3: Reply Codes in Numeric Orderrfc-editor.org
- RFC 5321, section 3.1: Session Initiationrfc-editor.org
- RFC 5321, section 3.3: Mail Transactionsrfc-editor.org
Related codes
- 5.5.1Invalid SMTP command.The sending software issued an SMTP command that the server does not support, or sent it at the wrong point in the conversation.
- 530Authentication required.The server requires the client to log in, or to start TLS, before it accepts this command, which usually means before it accepts any mail.
- 554Transaction failed.The server permanently refused the transaction, often after receiving the message data or straight away when the connection opened.
- 5.5.0SMTP protocol problem, unspecified.Something went wrong in the SMTP conversation between the servers that the receiving side did not describe with a more specific protocol code.
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