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Why do I get a “503 Valid RCPT command must precede DATA” message when I send to some people?
This mail message is sent to you automatically by the mail server. It most often happens when you are sending to someone who doesn’t use the same mail provider you use. The mail server will automatically send you this notification when you have not configured Outlook to send your login information to the server before sending e-mail. The body of the notification email will have a message that looks something like this:
FROM: System Administrator TO: InetDaemon Subject: Undeliverable: Test Message Your message did not reach some or all of the intended recipients. Subject: Test Message Sent: 6/2/2012 7:38 AM The following recipient(s) cannot be reached: <email@example.com> on 6/2/2012 7:38 AM 503 Valid RCPT command must precede DATA
Responsible email providers will require you to configure Outlook to log in to their server before sending e-mail to restrict the ability to send mail from their servers to just their own customers. This helps block spam by preventing anyone who doesn’t have a login from sending an e-mail from that server. Your mail service provider requires that you verify your account information before downloading mail (via IMAP or POP3), and they also require it before sending mail via Simple Mail Transfer Protocol (SMTP).
The mail server that actually sends your e-mails for you has been configured to use the SMTP AUTH extension as defined in RFC 2554. When sending an e-mail, the mail user agent (Outlook) connects to the mail transfer agent (your provider’s mail server) and the server responds with the authentication types used by sending the client the SMTP verb “AUTH” and a list of methods. The client selects an authentication method it supports and replies back with “AUTH <method>”. If the server does not receive an AUTH from the client, it treats the client as an unknown sender and does whatever the server is programmed to do when an unknown sender sends mail (ignore the mail message, send a warning message to the client).
How to fix “503 valid RCPT command must precede DATA”:
Note: The screenshots below are from Outlook 2010, but the settings that need to be changed are the similar in any version of Outlook.
NOTE: If your e-mail already works, you might want to uncheck the “Test Account settings by..” checkbox.