You send an email, and a few seconds later a bounce comes back saying 503 Valid RCPT command must precede DATA. The message didn’t arrive. Here’s what the error actually means and the causes to check, starting with the most common.
What the error means
Email travels between servers in a short, fixed conversation. The sending server says who the message is from, then who it’s going to (the RCPT command), and only then sends the message itself (the DATA command).
This error means the message body was sent before any recipient had been accepted. In practice, the receiving side refused the recipient, or something broke the conversation, and the message was sent anyway. The 503 is the symptom. The real reason is usually the recipient being refused earlier on.
It also isn’t really an Outlook error. It comes from a mail server, and Outlook just displays it. You can get the same thing in Gmail, Apple Mail or on your phone.
Two different situations
- The error appears the moment you click Send. This points to your own outgoing mail settings, usually SMTP authentication.
- You get an “Undeliverable” email a bit later. This means the message left your server and was refused further along. The cause is usually on the recipient’s side, or in between.
Knowing which one you have saves a lot of time.
Common causes
1. The recipient’s mailbox is full
This is one of the most common causes of the bounce-back version. If the recipient’s mailbox has run out of server space, their server can refuse the recipient, and you end up with the 503. We saw this recently when a forwarded email with a PDF attached bounced for exactly this reason: the recipient’s server space was exceeded.
A well-behaved server normally says “mailbox full” or “over quota” outright, but some servers and mail security gateways don’t, which is why the message you see is so unhelpful.
What to do: ask the recipient (by phone or WhatsApp) to clear space, or ask their IT or host to increase the mailbox size. In the meantime, send a share link instead of an attachment.
2. The address is wrong or no longer exists
A typo, a staff member who has left, or a disabled mailbox will all get the recipient refused. Check the spelling and confirm the address with the person.
3. SMTP authentication isn’t set up on your side
If your outgoing server requires you to log in and your email program isn’t set up to do that, the server refuses to send. See the Outlook steps below.
4. A firewall or antivirus mail scanner is interfering
Some firewalls and mail-scanning programs inspect SMTP traffic and can mess up the order of the commands. If the error happens with many different recipients, this is a good suspect.
5. Your sending IP address is blocked or blacklisted
If the other server doesn’t trust your IP address, it may refuse your mail. You can check this with a free blacklist lookup tool such as MXToolbox.
6. “Check mail before sending” (older setups)
Some older mail servers only allow sending shortly after you have checked for new mail (often called POP before SMTP). If you only get the error after your email program has sat idle for a while, check your inbox first and try again. Most modern servers don’t work this way, so don’t start here.
How to work out which one it is
- Read the whole bounce message. Look for words like mailbox, quota, over limit, blocked or user unknown. Often the real reason is sitting a few lines away from the 503.
- Send a short plain email with no attachment to the same person. If it works, the attachment or the mailbox size is the problem.
- Send to a different address, such as your own Gmail. If that also fails, the problem is on your side. If only one domain fails, it’s on theirs.
- Contact the recipient another way and ask them to check their mailbox space.
- Check your IP address on a blacklist lookup.
Fixing SMTP authentication in Outlook
Configuring SMTP Authentication in your email client requires you to edit your email account SMTP (outgoing mail) settings. Add your email address and password to the SMTP settings, and you won’t receive any more Outlook Error 503 Valid RCPT command must precede DATA messages.
How you set up SMTP Authentication depends on the email client you use. Here’s how to set it up in Outlook:
- Open Outlook.
- Open the File menu and select Account Settings > Account Settings.
3. In the Account Settings window, select the email account you wish to set up and click Change…
4. In the Change Account window, click More Settings…
5. From the More Settings window, select the Outgoing Server tab.
6. Check My outgoing server (SMTP) requires authentication.
7. Select Use same settings as my incoming mail server.
8. Open the Advanced tab.
9. In the Advanced tab, set the Outgoing server (SMTP) port to 465. In the Use the following type of encryption drop-down, select SSL.
10. Click OK.
11. On the Internet E-Mail Settings window, click Next.
12. Click Finish.
Should you require any assistance with the above, please feel free to contact us.