The “503 valid RCPT command must precede DATA” error has no single solution and can really cause problems when trying to send mail, particularly when using Microsoft Outlook.
While there is no single solution to this problem, here are some possible solutions:
Remove any apostrophe ('), quotation marks ('), or any type of non-alphabetic character from the subject of the email you are sending.
If you are using a VPN to browse and receive this error, disconnect.
Check that your email account is not full (that it has exceeded its space quota). You can check this in your control panel and remember that by avoiding leaving a copy of your messages on the server you will be saving a lot of space in your email account.
Use a different SMTP port: Use port 26 (instead of the default port 25) for the outgoing port. ADSL providers like Arnet usually block port 25 for sending SMTP mail. In this case you can use the alternative port 587 without encryption or 465 with an SSL encrypted connection.
Confirm that your email account is configured correctly in your email client.
SMTP Authentication: in SMTP Outgoing Server enable the option "My Outgoing Server Requires Authentication". Check the option "Use the following data" and expressly enter your email and the password assigned to that email account. In this option, don't forget to enable Outlook to remember your password.
You can find this in: -Tools / Accounts / Account Properties and within the email account review -More Settings / Advanced Properties. There you will find the checkbox “My server requires authentication”.
Make sure that the email from which you send is the same as the one you receive. (Do not set the FROM field different from the email used in the outgoing SMTP configuration).
Outlook PST File: Sometimes error 503 will occur if you have a corrupt PST file. Try to repair it or talk to your technician friend to try.
