Depends on what you mean by "download". Yes, reading the header or body
involves downloading from the server, and your system will cache what it
reads. But there's no impact on the server's copy. I'm not at my Mac, but
on many IMAP clients you can configure whether it immediately downloads the
entire message or just the header. (I use the latter mode on my Palm
Tungsten C PDA, for example, and manually download the rest if I really
want to read it.)
You can't read what's on the server without downloading it. But unlike the
normal configuration of POP mail, the server retains the information after
downloading it; until you tell it not to. Of course, you can configure a
POP client to leave the message on the server, as well.
The biggest difference with IMAP is that message status, including whether
you've read it, is on the server rather than in the client. When you
configure a POP client to leave mail on the server, and then connect from a
different client, all the mail you'd already read is "new", and you need to
deal with it again. With IMAP, the new client immediately knows that you've
already read it.
Depends on where you move it. Your mail folders can be on the IMAP server or
on your local system. Moving the message to a local folder will implicitly
mark it deleted on the IMAP server and download it to your local folder.
Moving the message to another IMAP folder will just update the server's
database without downloading anything.
Depends on how you've configured your client. If there's a Mail.app IMAP
configuration option to avoid downloading the message body until you read
it, then absolutely. Mozilla, for example, normally does NOT download the
message body until you read it; nor does the mail on my T|C.
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