You are receiving the error "503 Valid RCPT command must precede DATA", this means that the connection settings in your Outlook are not properly authenticated by your email server therefore you experience this error message.
The most common reasons for this error are:
a) Email server requires you to check email first before sending the email.
b) User's email client has mis-configured SMTP Authentication.
Please follow the steps below to correct the connection settings in your outlook.
1) First, go to "File" then select the respective account then click on "Account Settings" in Outlook.
2) Then choose your email account that you want to modify the email settings and click “Manage” or “Change” button.
3) Once you see the settings page in front of you, click "More Settings" and then on the "Outgoing Server" tab. Make sure that "My outgoing server (SMTP) requires authentication" is ticked and select "Use same settings as my incoming mail server
