10503 error when sending messages

> We are occasionally getting the following error when we are sending messages
> VPOP3 encountered a TCP/IP error when sending a message to the remote mail relay server "Our ISP".
> The last command that VPOP3 sent was ". <msg terminator>".
> The TCP/IP error was "10503 [No More Data]"

The 10503 error simply means that VPOP3 sent the message, got up to the 'Message terminator' (at the end of the message) and then the connection to the remote mail server was closed by the remote end.

Unfortunately, the error message from VPOP3 is rather uninformative, because it has no way of knowing why the connection was closed.

Any possible causes are pure guesses:
 - bug with the remote SMTP server
 - misconfiguration of the remote SMTP server
 - server load issue with the remote SMTP server
 - virus scanner or security software problem on the VPOP3 computer
 - problem with hardware firewall/router on your Internet connection if it does SMTP filtering of any sort

It is unlikely to be an Internet connection problem because those would usually give a 10053 or 10054 error when the connection is dropped unexpectedly. The 10503 error means that the connection was closed down properly, but just at an unexpected time.

As the problem is happening after VPOP3 has sent the message, it is likely to be something that is processing the sent message content, and is then failing.

The one thing the 10503 error tells you for certain is that the problem is not in VPOP3 itself, as the error only happens if the connection is closed from the remote end (from VPOP3's point of view)