Which of the following formats should be used to allow …

IMAP, especially the current version (IMAP4), is a more sophisticated protocol. It allows users to group related messages and place them in folders, which can in turn be arranged hierarchically. It’s also equipped with message flags that indicate whether a message has been read, deleted, or replied to. It even allows users to carry out searches against the server mailboxes.

Here’s how IMAP works in a nutshell:

Connects to the mail server on port 143 (or 993 for SSL/TLS connections);
Retrieves email messages;
Stays connected until the mail client app is closed and downloads messages on demand.

There’s one good reason why IMAP was designed to store messages on the server. It’s meant to enable retrieval of messages from multiple devices; sometimes, even simultaneously. So if you have an iPhone, an Android tablet, a laptop, and a desktop, and you want to read email from any or all of these devices, IMAP would be the better choice.

If you access email messages from multiple devices (who doesn’t these days?), you’ll likely want all devices to reflect whatever action you performed on one device.

For instance, if you read messages, A, B, and C, then you’ll want those messages to be also marked as “read” on the other devices. If you deleted messages B and C, then you’ll want those same messages removed from your inbox on the other devices as well. If you moved message A to another folder … well, you know what I mean. All these synchronizations can only be achieved if you’re using IMAP.