How email works ( greatly oversimplified basic guide.)
Email has two parts: incoming and outgoing. Incoming is all the email that someone else sends to you, and outgoing is all the email you’re sending to someone else.
Incoming is handled by a machine out on the internet. Outgoing is handled by a different machine out there on the internet. Neither of these are your computer. They usually belong to whoever is providing your internet service; your Internet Service Provider, or ISP. That’s who you are paying to be on the internet in the first place: ATT, Comcast etc.
Each of these two, incoming and outgoing, require that you have an account, and a password, else anyone could claim to be you sending outgoing mail, or anyone could claim to be you and read your incoming mail.
So, when you sit at home and set up your email program, it will require both these accounts and their information.
The OUTgoing machine (called a server, because it is serving you with the ability so send emails) is denoted by “SMTP” (OK, if you must know: Simple Mail Transfer Protocol.) You put in your name and password, and when you hit “send” on an email to a friend, it is sent to that SMTP server, which does all the work of routing it to…
… an INcoming server. (See? A different machine altogether.)
When someone sends you an email it goes to your ISP’s INcoming email server, (which is just a computer with a hard drive, nothing special) and is saved on the server’s hard drive. It just sits there waiting for you to retrieve it by signing in and asking for your saved email.
(Most of us have our email set up to do this automatically, and periodically, so we don’t have to jump thru sign-in hoops every time we want to check for new incoming email.)
Fine. NOW… there are two ways for your ISP’s server to respond to your request to send you the email that is waiting on their hard drive: 1) Send it, and immediately delete it from the server hard drive; or 2) send it and -leave it- on the server hard drive.
These are “POP” (send and delete) or “IMAP” (send and leave it there.)
That’s basically it.
Here’s a tiny bit more, however. 1) IMAP is widely used bacause if you leave the mail on the server, you not only have a backup, but you can retrive it using another device (cell phone; laptop; iPad.) You cannot do that with POP because once you have retrieved it, it’s deleted from the server, and the only place it exists now is your own computer.
IMAP will also automatically send you your email as it arrives. POP requires that you log in and ask for it (generally speaking.)
A discussion-list, which you join with a bunch of other people, is really an “automated” SMTP server. You send it a message, and it simply forwards that message on to the email addresses of everyone on the list.