The Reader’s Question:

Dear InetDaemon,

Why do I get a “503 Valid RCPT command must precede DATA” message when I send to some people?


The Error Message

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:
      <address@domain.tld> on 6/2/2012 7:38 AM
             503 Valid RCPT command must precede DATA

Why It Happens

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).

Sending login information to the server before sending an e-mail is not a Microsoft Outlook default, so you get an error from your email provider’s mail server when sending to external providers.

The Geeky Bits

The mail server that actually sends your e-mails for you has been configured to use theSMTP 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 It

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.

  1. Click the File tab:Outlook 2010 File Tab

    Outlook 2010 File Tab


  2. Verify that the e-mail account shown at the top under “Account Information”  is correct. Click the Account Settings selector button, then select the Account Settings drop-down option.Outlook 2010 Account Settings

    Outlook 2010 Account Settings


  3. Select the correct account (if you have more than one)
    Click Change 

    Outlook 2010 - Account Settings - Change

    Outlook 2010 – Account Settings – Change


  4. Click the More Settings button:Outlook 2010 - Account Settings - More Settings

    Outlook 2010 – Account Settings – More Settings

    NOTE: If your e-mail already works, you might want to uncheck the “Test Account settings by..” checkbox.

  5. Click the Outgoing Server tab.
    Click the checkbox for My outgoing server (SMTP) requires authentication.
    Select  Use the same settings as my incoming mail server radio-button option. 

    Outlook 2010 - E-Mail Settings - Outgoing Server

    Outlook 2010 – E-Mail Settings – Outgoing Server Settings


  6. Click OK
  7. Click Next. Outlook will try to send a test message back to yourself, unless you unchecked the “Send a test message” checkbox.
  8. Click Finish
If Outlook is configured to send login information for outgoing mail and you’re still getting this error, contact your ISP or mail provider for support.

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