The 503 Valid RCPT commands must precede DATA error in Microsoft Outlook is a fairly common error message.
We list the most common causes and solutions below:
1. Ensure that you have enabled authentication for outgoing email.
2. Remove any apostrophes (single quotes) or other non-alphabetic characters from your e-mail account name(s).
3. Check your outgoing mail settings. We must enable authentication for outgoing email. If you are sending through port 25, change the outgoing port to 587.
4. Ensure that you are sending from the same e-mail address that the recipient sees your e-mail coming in as (i.e. The From field should not differ from the e-mail address used in your outgoing mail settings).
5. An antivirus software can also interfere with the sending of mail. Test by disabling the mail scanning feature within your anti-virus software and then restarting Outlook.
6. Sometimes the 503 error will occur if you have a corrupt Outlook data file (also referred to as the PST file). If the file is corrupt, it will prevent any new messages from being stored (whether you are downloading or sending mail). Running Outlook's Inbox Repair Tool is the best way to check the file.
Should none of the suggestions above correct the problem, then there may be an issue with your Outlook installation. You may need to contact a technician to rectify the issue.