How to solve error 503 valid RCPT command must precede DATA

Problem:
Your email client generates the following error: 503 valid RCPT command must precede DATA

Reasons:
This error usually occurs because of one of the following issues:

• A full cPanel mailbox
• Incorrect email address / typing error in your email address or username
• Your email client is not authenticating.
• Your email address your authenticating for and the email address you’re actually using do not match
• Your Outlook PST file has errors

Solutions:

• Login to cPanel, go to Mail Account and then ensure that the mailbox related to your error is not full
• Check your email values in your email client
• Ensure that your email client is set to use authentication for outgoing email
• Make sure you’re sending from the same email address that the recipient sees your email coming in as (ie. don’t make the “from” field differ from the email address used in your outgoing mail settings).
• Try repairing your Outlook file using scanpst.exe