What's happening to my Mail?

Question

I have a two IMAP email accounts (one is Gmail) set up conjunctively on my iMac, MacBook Pro, and iPhone. I have all devices set up to iCloud.
I'm not understanding how the three devices are working together and it seems that some of my mail, once viewed, is disappearing. Can someone explain how IMAP mail works? And if I view the mail on one device, why does it disappear from the other devices, even though I haven't deleted it?
And lastly, is there an easy-to-understand online instruction of the latest versions of Mail?

Answer 1

First, third-party (non-Apple) mail accounts have nothing to do with iCloud. iCloud does not sync other mail accounts through its servers.
IMAP provides its own syncing, and when you access an IMAP server through your mail program on a device or computer you're essentially looking at the mail on the server itself (although most mail clients download a copy of the mail so you can view it faster and see it when you're offline). When you read a message on one device, that "read" status is sent back to the server, and any other devices which access it (eg, your iPhone) make a note of that status and mark the mail as "read" (so the blue unread blob will disappear). Likewise, when you delete or move a message to a folder (IMAP servers can store mail folders, unlike POP servers), all devices are updated with this new status.
So that doesn't really explain why a message would disappear once you read it - it should simply be marked as read. Did you definitely set all devices up as IMAP, and have you used the recommended client settings in this article?
https://support.google.com/mail/answer/78892?hl=en
There is some help available in the Help - Mail Help menu, but you might also search Youtube for Mountain Lion Mail tutorials. The Apple tutorial here actually shows the Lion version.
Matt