Error 503: Valid RCPT command must precede DATA (Thunderbird)

Error 503 Valid RCPT command must precede DATA can occur when sending an email. It’s telling you that your email server did not properly authenticate your SMTP (outgoing mail) connection.

The two most common causes for this error are:

  • Your email server requires you to check email first before sending an email (checking email first is one way your email provider manages the security of your email account).
  • Your email client isn’t set up for SMTP Authentication.

Checking email before sending

When you first open your email client, it automatically checks your email first. However, if you attempt to send an email after about 30 minutes and see this message, simply check your email for new messages. You’ll then be able to send an email if this is the problem (most common with Outlook).

More often, Outlook Error 503 tells you that you need to set up SMTP Authentication in your email client.

Setting up SMTP Authentication

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 Thunderbird (V52):

  1. Open the Tools menu and select Account Options.
  2. Scroll to the bottom of the Accounts panel and click Outgoing Server.
  3. The Outgoing Server (SMTP) Settings display in the right panel. Select the SMTP server you wish to change and click the Edit button.
  4. The SMTP window opens.
  5. In the SMTP window, set Connection security to SSL/TLS, Authentication method to Normal password, and enter your email address in the User Name field. 
  6. Click OK to save your settings.

The first time you check your email, you’ll be asked for your account password. Enter your password and click the Save Password box to save your password and avoid re-entering it in the future.

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