The diagram below depicts the entities involved in sending and receiving email.
Email receivers decide whether your email gets delivered. They comprise the entire federation of network systems, software, and policies that manage email delivery. There are several different classes of email receivers, and you must know the class of receiver you’re sending to in order to ensure that you’re optimizing your email program for deliverability.
An ISP (Internet Service Provider) usually hosts email services for subscribers. Typically, B2C (business to consumer) emails are sent to addresses hosted at ISPs. Sites like Yahoo!, Gmail, and Comcast fall under this category. These large providers make decisions across millions of mailboxes about whether your email is spam, and they base these decisions on many recipients’ feedback.
Most often, this feedback comes in the form of a complaint – when a recipient marks a message as spam in their email client – but it can also include whether recipients are opening or clicking on your email. The amount of recipient feedback at an ISP’s disposal for reputation calculations is usually vast, but ISPs tend to implement a “one size fits all” reputation system that they can easily roll out across all mailboxes.
A corporate system usually refers to the independently-hosted email system used by employees, students, governmental agencies, or certain paid or non-profit associations. This system is commonly referred to as a B2B (business to business) system. A corporate system also can include the hosted email services that ISPs offer to run inbound email systems. Often the rules for spam prevention are set by the in-house Information Technology (IT) department, or the rules are defaults provided by the email-receiving Mail Transfer Agent (MTA) solution, like Microsoft Exchange.