After sending email messages I frequently receive the following message: ‘550 #5.1.0 Address rejected’. This seems to be a widely reported problem, but there are no real answers on how to fix the problem.

The ‘Address Rejected’ message could appear for a few reasons, depending on how your ISP has configured their mail server. The first reason is if your ISPs SMTP (outbound) mail server cannot authenticate your identity, and is unable to tell whether you are allowed to send email through the sever. Many ISPs lock-down their mail servers heavily, to prevent people from outside their network (i.e. users who are not connected through the ISP) from using their SMTP server. This is done to prevent people from sending spam through their servers. The second reason is the SMTP server has detected something wrong with the email address to which you are attempting to send email. This could include specifying an invalid email address, or even the SMTP server being configured to not send email to a particular email domain. In either case, since the message is coming from the SMTP server there is not much we can do to resolve the problem, especially since I do not know the particulars of your setup or the mail server configuration. Therefore, your best option is to contact your ISP and ask for help resolving this problem, as they will be most familiar with the required setup for their services.