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For those that are getting the "503 valid RCPT command must precede DATA" error (we'll refer to it as simply the 503 error going forwards) when trying to send email, here are some common solutions that will hopefully get you back up and running.

Check your outgoing SMTP address - if you are using an old or outdated smtp address some of your contacts will reply with this error. To find out what your latest smtp address is, contact your ISP or IT support. Alternatively you can find out your current address by googling for 'smpt address' along with the company name that supplies your email.

Apostrophes - remove any apostrophes (single quotes) or other non-alphabetic characters from your email account name(s).

VPN - 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.

Full mailbox - 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."

SMTP port - use port 26 (instead of the default 25) for outgoing mail.

Server authentication - setup 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.

Sender matching - make sure you're 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 - sometimes the 503 error will occur if you have a corrupt outlook PST (mail archive) file. Try repairing the file. This can be done by running the scanost or scanpst file which automatically installs with Outlook.

Most of this information was gained from the werockyourweb site (thanks to them) which can be found here; http://www.werockyourweb.com/503-valid-rcpt-command-must-precede-data