What does 503 mean?

SMTP 503 indicates commands sent out of sequence:

What it means:

You sent a valid command at an invalid time. SMTP requires commands in a specific order. The server expected something different.

Required SMTP sequence:

EHLO/HELO first. MAIL FROM before RCPT TO. RCPT TO before DATA. Protocol must be followed.

Common causes:

Skipping steps: Trying RCPT TO before MAIL FROM. Repeating commands: Sending EHLO mid-transaction. Software bugs: Mail client not following protocol.

Typical message examples:

"503 Bad sequence of commands" "503 Need MAIL command" "503 RCPT first"

Response:

Review and fix your SMTP implementation. Commands must follow the required order.

503 says "you're out of order." Follow the protocol sequence.