What can I do about the error message "Host did not acknowledge DATA command [554. 5.5.2 No valid recipients]"?

If you are using Exchange 2000 Server you might want to check the following settings:

1. Are all recipient domains you want to support added to the default policy in the recipient policies in your Exchange configuration? Check out our Exchange 2003 and  Exchange 2000 configuration guide, Part 1. Make sure that the check mark left of the domain in question is there as well.
2. The relay rights in the virtual default SMTP connector: You need to give the machine that runs POPcon (even if it is the same machine that runs Exchange 2000 Server) RELAY RIGHTS. This is because often your ActiveDirectory domain will not be the same as your email domain (name).