503 Valid RCPT command must precede DATA
You will get that error message if your mail client is not properly configured. To be exact, you missed to enable the option “My SMTP server requires authentication”. Once you enable it you will not see the error anymore. Check our guide how to configure your email address properly by visiting https://www.scalahosting.com/kb/configure-email-microsoft-outlook
Your firewall/antivirus system may cause that error to appear as well. If the error persists after enabling SMTP authentication in your mail client, try disabling your firewall/antivirus.
Why SMTP authentication is required?
If ScalaHosting mail servers do not require authentication spammers will be able to use them to send SPAM. Only authorized users can access the mail server to send messages. We do not allow any kind of open relays. In that way we keep our mail server IP’s clean and with good reputation.
Useful tip: ScalaHosting recommends connecting to all mail services via SSL. In that way you increase the security of the communication between your mail client and the server. All data is encrypted and noone can steal it.