of email messages may be downloaded, but fundamentally, IMAP provides what can best be termed a window or a view on a collection of email stored on the server.
While copies of email may be downloaded, enabling offline access, the IMAP protocol works best when continuously connected to the email server. See also: SMTP, the protocol for sending email, and POP3, the protocol for downloading mail.
Check out my article How do Gmail labels relate to folders? In my opinion, this is an under-appreciated feature of IMAP.
It’s generally recommended that important data be backed up in data also be backed up in another physical location, such as a different building, or online.
More than anything, the important concept is that there never be only a single copy of important data. See also: back up the email from the email server to your PC.
For example, your email program may very well actually download a copy of all newly arrived email to your PC. You’re still looking at your email as it lives on the mail server’s repository, but your email program has optimized the experience by downloading the email so it can be accessed and displayed more quickly. When IMAP downloads your email, it is of data, ideally kept in a different location than the original.
In fact, email downloaded by IMAP can be examined off-line, if your email program is appropriately configured. If there’s only one copy of something – say a photograph on a mobile phone – then it’s not backed up.
It can also be supported by stand-alone email programs on other devices.
When POP3 is used, email messages are from the email server on which they were stored.
So if something happens to the master copy – say an email is deleted, or marked as “read” – then that change will be reflected in all the email programs. Mark it read there, it’ll show up as read here: cross-device synchronization. What that means is that if you create a folder on one machine connected to your email account using IMAP, then that folder becomes visible in all email programs connected to that email account via IMAP.
And, of course, if you move a message into a folder, that message is moved into the folder in all email programs connected to that account. Gmail actually doesn’t support folders at all, but instead provides roughly equivalent functionality through the use of labels.
File-transfer protocols might specify nothing more than the name of the file and its contents, once again in a very formal and rigorous way.
Protocols of various kinds define almost all computer-to-computer and program-to-program interaction and information exchange. POP3 is used by email programs like Thunderbird, Microsoft Office Outlook, and others use to communicate to mail servers when downloading your email.
See also: SMTP for sending mail, and IMAP for accessing email without removing it from the email server.