Untuk mengatasi masalah ini, anda harus melakukan configurasi email client (outlook, dll.) menggunakan authentication smtp server dengan memilih “My server requires authentication” pada box pilih configurasi di email client anda.

Berikut tutorial dalam bahasa Inggris:
Error 503: Valid RCPT command must precede DATA

This means you must check your email before trying to send mail meaning that our mail server requires authentication, which can be achieved using one of these methods:

A) Pop before smtp — check your email 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 that IP address for up to 30 minutes. After 30 minutes you have to authenticate again.

B) Configure your email client to authenticate automatically by checking the “my server requires authentication” box in your email configuration. You will be able to send mail without having to “pop” your mail box first.

If using MS Outlook, please try the following:

1) Open outlook
2) Go to Tools/Email Accounts
3) Select the Account with the trouble and click Change
4) Click on More Settings
5) Click on Outgoing Server Tab
6) Click “Outgoing Mail Server (SMTP) requires authentication” and make sure
“Use same settings as incoming mail server” is selected (see attached image)
7) Click on “Advanced Tab”
Set outgoing Port (SMTP) to 26 (if your hosting account is Linux) or 587
if your hosting account is Windows.
9) Click Ok and then Test Account Settings

If it fails on send then repeat starting from 7.

7) Click on Advanced Tab
Set outgoing Port (SMTP) to 465 (See attached)
9) Check the box Beside “The server requires an encrypted connection (SSL)”.
10 ) Click ok and then Test Account Settings