This POP before SMTP policy can create some problems for subscribers using a proxy server. The new IP address is not in the tenet database, so the mail is rejected. There are TWO possibilites. Your mail program checks mail before sending mail. Your mail program sends mail before checking mail.
Mail program tries to send mail.
As mentioned above, the mail server will remember that you are allowed to send for two hours. This means that the first time you try to send and receive, you might get an error message. All subsequent attempts should go through for the next two hours. After those two hours, you would have to start the process over again. The University of Texas at Austin.