Check your mail 2. We have had a lot of problems with people who want to send spam through our mail servers. Usually they are not customers. We have blocked all mail sending from offsite to reduce spam. A big problem with mail sending is that the mail server doesn't check passwords, it only asks for a password when you are trying to receive your mail.
Security is abysmal on standard sendmail.
So, any bozo could potentially send 10 million emails from any place on the Internet through our server and it would be impossible for us to control. Yes, people have tried this before. Yes, they are in jail now. But by using mail checking to approve your password, it gives us a way to authenticate and identify real customers so that you will be allowed to send mail no matter where you are. This software is something we wrote up in house.