In 1999 I wrote a series of Border Gateway Protocol tutorials including a tutorial on autonomous system numbers. Since then AS numbers have been changed from 16-bit to 32-bit numbers to avoid running out of identifiers for BGP sessions. I have updated theAS numbers tutorial with a table that outlines what each range of autonomous system numbers are used for.

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All of a sudden, you can’t send or receive e-mail.


Your mailbox may be full.

Your computer doesn’t actually send and receive e-mails directly.  Your mail client software, either Microsoft Outlook, Mozilla Thunderbird, SeaMonkey, Apple Mail etc.  will send and receive mail via the Internet’s version of the local post office, your Internet service provider’s mail server.   Your provider sets aside storage space on their servers to store mail while you are not connected to it (when your mail software isn’t running).  Most providers have a limit of how much mail you can store on their servers, and the mail stored on their servers.  If your provider offers webmail as a service, only the mail stored at the provider’s servers will be visible in your mailbox.

When this storage space at the servers is used up (full) you cannot send or receive any more e-mail until you delete something.  It’s more or less the same as having a post office box at the post office, and the box being too full to cram any more letters into.  Periodically, you have to go to the post office and empty it.  Big parcels will fill the box faster than small letters.  With e-mail, the big parcels are e-mails with attachments, or e-mails with pictures or video embedded in them. The bigger the mail messages are, the faster the mailbox fills up.  Consider carefully what you send and receive.

Let me show you how to fix this…

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(Jump straight to The Fix)

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:
      <[email protected]> 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 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 It

How to fix “503 valid RCPT command must precede DATA”:
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Cool flash applet HTML5/JavaScript page showing the history of the development of web standards and web browsers:

(I stand corrected.. 😉  )