This error message:
The server responded: 503 valid RCPT command must precede DATA
This means you must check your email before trying to send mail. The mail server requires authentication (SMTP AUTH), you will need to configure the email accounts within your email client to authenticate automatically by checking the "my server requires authentication" box in your email configuration.
POP before SMTP - check your email account for new mail before trying to send email. This adds your IP address to a relayhosts file and allows any email to be sent from your IP address for up to 30 minutes.
After 30 minutes you have to authenticate again, you may also have to authenticate again if you are disconnected from your dial-up connection. If you are using Outlook then under Email Accounts within the more settings for SMTP you can select always check email first.
Configure your email client to authenticate automatically, if you are using MS Outlook, please try the following:
Goto Tools/Email Accounts
Select the Account with the trouble and click Change
Click on More Settings
Click on Outgoing Server Tab
Click "Outgoing Mail Server (SMTP) requires authentication" and make sure "Use same settings as incoming mail server" is selected
Click on "Advanced Tab"
Set outgoing Port (SMTP) to 26
Click Ok and then Test Account Settings