because it simple...

How email works, what is email

Main content page - How email works.SPF and SenderID work by having the domain publish what might be a sizeable list of IP addresses that identify MTAs they authorize to carry their email. This works as a type of path registration, but does not scale well since it also combines both IPv4 and IPv6 addresses.
The first part of the address, the email user name, identifies a unique user on an email server. The @ symbol separates the user name from the host or server name. The host name uniquely identifies the server computer and is the last part of the Internet email address.
Email hosting services thus differ from typical end-user email providers such as webmail sites. They cater mostly to demanding email users and Small and Mid Size (SME) businesses, while larger enterprises usually run their own email hosting service.
This means the email is delivered faster that waiting to retry the primary server until it recovers. Some front end spam filtering services apply gray listing techniques only on the primary server to discourage spam bot email.
An email hosting service is an Internet hosting service that runs email servers. Email hosting services usually offer premium email at a cost as opposed to advertising supported free email or free webmail.