Mail Transfer Agent’s (MTAs) communicate with each other over the internet using SMTP protocol (SMTP servers). The recipient’s MTA then forwards the email on to the incoming mail server (MDA, mail delivery agent) which is tasked with storing the mail until the user accepts it. To retrieve email on an MDA, a supporting protocol must be used. There are two main protocols, POP3 and IMAP. You might recognize these two acronyms since incoming mail servers are called either POP servers, or IMAP servers depending on which protocol they use.
Pop vs. IMAP
POP stands for Post Office Protocol. This piece of software is used for retrieving email. Just like a visit to a post office you can drop in, pick up your mail and leave. POP3 gives an email user access to their emails stored in their user account on that server. You don’t need to stay online for the emails to come through. You just need to leave a copy of an email on the server to access it.
POP does have some drawbacks; namely, information transmitted through POP travels one way. This means that once an email is downloaded to a client, the client takes charge of sorting through the different status flags (e.g. sent, deleted or answered). This was fine when the internet was young, before smartphones, tablets and the like. People accessed their mail from a single location. Nowadays, it’s more likely that you access emails from many places; thunderbird at home, the mail app on your cell phone or a web interface when you are at work for example. With POP, you would have to sort through the information over each different device, that is assuming you’ve saved a copy of each email on the server, to begin with.
IMAP (Internet Message Access Protocol) is a bit smarter about how it coordinates emails. IMAP clients have two-way communication with their servers. The IMAP protocol saves a copy of every message on the server so that unlike with POP, multiple clients can access them. It’s completely synchronized. With IMAP, when you check an email on your tablet, it will be marked as read when you check your inbox on your phone. This happens because the status of the email is updated with all other clients during the server interaction.
Back to the post office analogy. IMAP is just like when your mail is categorized and stored at the post office for you and redelivers it when you are at home, at work, or pick it up there in person. You can keep a properly marked archive on your home client as well as on your mail server. IMAP has an offline mode where any changes are synced with the server the next time you’re online. You may configure IMAP mail servers to fetch mail from POP inboxes, too, which works well if you’re seeking to consolidate. Of course, given that IMAP works with the "cloud" best, servers get entry too, and storage can be problems. Thankfully, storage space and bandwidth isn’t as pricey as it once was, but this will truly be a change-off for a few humans.