How email works.
This is a rough and ready explanation on how email is supposes to work, and should also help enlighten why mail gets rejected or denied on the internet. ( other than from typing errors or specific blocking )
Lets take the first three scenarios of how you get an email address:
1. When you connect to the internet through an access provider you are given an email address at the provider who gives you connectivity. This email address you are given is setup in his mail server. We will refer to this as the "Access email."
2. When you get a Domain you get email addresses at the Domain. These email address you are given is setup in the Domains mail server. We will refer to this as the "Domain email."
3. When you sign up for a web email system, you get email addresses at that domain that provides the web mail service. This email address you are given is setup in the web mail server service. We will refer to this as the "Web email."
Fact. 1. All email addresses that you get are setup so that the email address is linked to the specific mail server and the mail server IP, where you have the address, that responds by receiving and sending the email. This implies that the mail server that is setup up for your specific email, will and should only send and receive, for your specific email address.
This in turn establishes that there are three separate ways to send and receive email.
Most mail servers today have the ability to verify that the email being received is coming from the actual Domain name and IP address that is setup to send and receive the mail. This is done by a process called reverse lookup. This is the established norm and is the correct documented way for email to work and to establish that the email is coming from the verifiable mail server located at the specified IP address in the reverse lookup
Fact. 2. Most mail is rejected due to laziness or ignorance on the part of the sender, plus the misconfiguration of many ( most ) mail servers by their Administrators, whom may also be ignorant as to how email is supposed to work. If a mail server is misconfigured and does not have the proper reverse records in DNS, mail can and will be rejected.
Fact. 3. The misconception is common that when an email is sent from the clients email program it will be delivered. The Clients email program is configured with a specific mail server, and should only allow the specific email address to be used from that mail server. When a mail server allows an email address which it is not configured for, to be sent, its misconfigured. Commonly referred to as an Open Relay or Relay.
In summary, we determine that a "web mail", should not be expected to be delivered from a "Domain email" or an "Access email" mail server ( as it should not be configured to do so ) as well as the other way around. i.e. If everybody understood, how email works and applied due diligence, on how they use email, junkmail would also be reduced.
Please note mail servers send to other mail servers, which are configured with the email account at the domain details. All mail servers are expected to have a reverse DNS entry, to verify the sender.