SMTP commands were sent in the wrong order. The SMTP protocol requires commands in a specific sequence: HELO/EHLO, then MAIL FROM, then RCPT TO, then DATA. Sending commands out of order triggers this error.
What Does Error 503 Mean?
SMTP code 503 indicates that your sending server issued commands in an incorrect order. The SMTP protocol requires a specific sequence: first EHLO/HELO to identify your server, then MAIL FROM to specify the sender, then one or more RCPT TO for recipients, and finally DATA to transmit the message content.
This error can also occur if a command is issued before the required authentication step. For example, attempting MAIL FROM before AUTH on a server that requires authentication will return 503. Similarly, sending DATA before any RCPT TO commands, or issuing MAIL FROM twice without completing the first transaction, will trigger this error.
Common Causes
- Sending DATA before completing MAIL FROM and RCPT TO
- Issuing MAIL FROM before HELO/EHLO
- Attempting to send without required authentication (AUTH)
- Sending duplicate commands without completing the transaction
- Software bug in the sending MTA causing out-of-order commands
How to Fix Error 503
- Review your MTA configuration to ensure proper SMTP command sequencing
- Ensure authentication (AUTH) is completed before MAIL FROM if required
- Check for bugs or misconfiguration in your sending software
- Test the SMTP conversation manually using telnet or openssl to identify the sequence issue
Frequently Asked Questions
SMTP error 503 means your email client sent an SMTP command out of the required order. The SMTP protocol requires commands in a specific sequence: EHLO, then AUTH (if required), then MAIL FROM, RCPT TO, and finally DATA. If any command is sent before its prerequisite -- such as attempting to send data before specifying the recipient -- the server rejects it with a 503 error.
First, enable SMTP authentication in your email client settings by entering the correct username and password. Verify your SMTP server address and port number (typically 587 for TLS or 465 for SSL). Ensure the sender email address in the "From" field is valid and properly formatted. If using custom code, confirm commands follow the correct sequence: EHLO, AUTH, MAIL FROM, RCPT TO, DATA. Use EHLO instead of HELO to enable proper authentication support.
In most cases, yes. The most common cause of SMTP 503 is a missing authentication step. Modern mail servers require valid credentials (username and password) before allowing you to send email. If your client attempts to issue MAIL FROM or RCPT TO commands without first authenticating via the AUTH command, the server responds with 503 "Bad sequence of commands." Enable the "My server requires authentication" option in your email client's outgoing server settings.