I have a couple of users who keep getting a 503 valid RCPT command must proceed data error in outlook. After checking all settings, deleting and recreating the account, deleting the profile, and reinstalling office to try and fix the problem. I came to find out that the error only happens when they are connected to the internet through Verizon or Armstrong Cable. If they connect through SBC, Cox, Qwest and any other ISP they can send emails with no problem.
The SMTP server is using port 587 and they can telnet into the SMTP server on this port. So it's not a case of the ISP blocking those ports.
After searching here and anywhere else i can think of all i could find was to make sure outlook was authenticating. But because we know the settings are correct and that the same account works on other networks that isn't the issue here.
The funny part is that the users in question use IMAP and when they get the send error it goes into the personal folders inbox instead of the users email account inbox.