Error 503: Valid RCPT command must precede DATA
There could unfortunately be a many causes of this error message.
Note: it is a case of working through the possible causes and solutions listed below and then attempting to re-send your email message.
Remove any apostrophes (single quotes) or other non-alphabetic characters from your email account name(s).
If your system is trying to connect via a VPN network while you're sending email, you may get this error. Try disconnecting the VPN before sending/ receiving mail.
Make sure the mailbox of the affected account is not full or has met/ exceeded its quota. You can uncheck the following option to make sure a copy of all your emails is not being left on the server (note that since the email will be downloaded to your PC after this setting change, you won't be able to access your emails from another location if you do this). In Outlook, browse to Tools > Account Setting > More settings > Advanced > uncheck "leave a copy of this message on the server."
Use port 26 (instead of the default 25) for outgoing mail.
Set up your outgoing mail to use outgoing server authentication (sends mail by logging in with your email address and password). In Outlook, browse to Tools > Account Settings > More settings > Advanced and change the outgoing server to 'Auto.' Next, browse back to More settings > Outgoing server and check the "My outgoing server uses the same authentication as my incoming server" box. If you select to have Outlook "remember your password" you won't have to type it in every time you send an email.
Make sure you are sending from the same email address that the recipient sees your email coming in as (ie. don't make the "from" field differ from the email address used in your outgoing mail settings).
Outlook PST files
If you are using Outlook, sometimes the 503 error will occur if you have a corrupt outlook PST (mail archive) file. Try repairing the file.